Fixing link to Android build guide.

Summary:
While browsing the repository, I noticed that the link to how to build for Android has changed. It's a very simple fix.

Verifying that the URL is correct.

None.

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[DOCS] [BUGFIX] [ReactAndroid/README.md] - Incorrect URL
Closes facebook#19450

Differential Revision: D8178077

Pulled By: hramos

fbshipit-source-id: 388c35b5322416333681d66ef1b8ca42e9e83151

ReactAndroid/README.md

@@ -1,6 +1,6 @@
 # Building React Native for Android
 
-See the [docs on the website](https://facebook.github.io/react-native/docs/android-building-from-source.html).
+See the [docs on the website](https://facebook.github.io/react-native/docs/building-from-source.html#android).
 
 # Running tests