Fix links in blog Atom feed

[hramos]

The root URL was missing the baseURL portion. This is fine when a site is hosted on a custom URL such as docusaurus.io, because the generated URL will be valid (docusaurus.io/blog), but it breaks when the site is hosted on GitHub Pages (the React Native feed was using facebook.github.io/blog as the rootURL).

The current React Native feed looks like this (truncated):

<feed xmlns="http://www.w3.org/2005/Atom">
<id>https://facebook.github.io/blog</id>
<title>React Native Blog</title>
<updated>2018-01-18T06:00:00Z</updated>
<generator>Feed for Node.js</generator>
<link rel="alternate" href="https://facebook.github.io/blog"/>
<subtitle>
The best place to stay up-to-date with the latest React Native news and events.
</subtitle>
<logo>https://facebook.github.ioimg/header_logo.png</logo>
<entry>
<title type="html">
<![CDATA[
Implementing Twitter’s App Loading Animation in React Native
]]>
</title>
<id>
https://facebook.github.io/blog/2018/01/18/implementing-twitters-app-loading-animation-in-react-native.html
</id>
<link href="https://facebook.github.io/blog/2018/01/18/implementing-twitters-app-loading-animation-in-react-native.html"></link>
<updated>2018-01-18T06:00:00Z</updated>
<summary type="html">
<![CDATA[
Twitter’s iOS app has a loading animation I quite enjoy. <img src="/react-native/blog/assets/loading-screen-01.gif" style="float: left; padding-right: 80px; padding-bottom: 20px"/> Once the app is ready, the Twitter logo delightfully expands, revea
]]>
</summary>
<author>
<name>Eli White</name>
<uri>https://github.com/TheSavior</uri>
</author>
</entry>

Note that the id at the top is incorrect (https://facebook.github.io/blog), as well as the individual entry links (https://facebook.github.io/blog/2018/01/18/implementing-twitters-app-loading-animation-in-react-native.html).

After this change, the URLs should result in https://facebook.github.io/react-native/blog and https://facebook.github.io/react-native/blog/2018/01/18/implementing-twitters-app-loading-animation-in-react-native.html respectively.

Docusaurus's own feed is fine, because it's base URL is a plain / as it is hosted on its own domain.

Test Plan

Generate feed in react-native-website, confirm links have been fixed:

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
    <id>https://facebook.github.io/react-native/blog</id>
    <title>React Native Blog</title>
    <updated>2018-01-18T06:00:00Z</updated>
    <generator>Feed for Node.js</generator>
    <link rel="alternate" href="https://facebook.github.io/react-native/blog"/>
    <subtitle>The best place to stay up-to-date with the latest React Native news and events.</subtitle>
    <logo>https://facebook.github.io/img/header_logo.png</logo>
    <entry>
        <title type="html"><![CDATA[Implementing Twitter’s App Loading Animation in React Native]]></title>
        <id>https://facebook.github.io/react-native/blog/2018/01/18/implementing-twitters-app-loading-animation-in-react-native.html</id>
        <link href="https://facebook.github.io/react-native/blog/2018/01/18/implementing-twitters-app-loading-animation-in-react-native.html">
        </link>
        <updated>2018-01-18T06:00:00Z</updated>
        <summary type="html"><![CDATA[Twitter’s iOS app has a loading animation I quite enjoy.

<img src="/react-native/blog/assets/loading-screen-01.gif" style="float: left; padding-right: 80px; padding-bottom: 20px"/>

Once the app is ready, the Twitter logo delightfully expands, revea]]></summary>
        <author>
            <name>Eli White</name>
            <uri>https://github.com/TheSavior</uri>
        </author>
    </entry>

[hramos]

Noticed that had the same issue. Fixed in previous commit.

[JoelMarcey]

Thank you @hramos!