doc: fix gitter badge in README

GitHub now renders Markdown in CommonMark where spaces in a link
destination are invalid and need to be escaped.

PR-URL: nodejs/node#11944
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

README.md

@@ -1,6 +1,6 @@
 # Node.js
 
-[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/nodejs/node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/29/badge)](https://bestpractices.coreinfrastructure.org/projects/29)
+[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/nodejs/node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/29/badge)](https://bestpractices.coreinfrastructure.org/projects/29)
 
 Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. Node.js
 uses an event-driven, non-blocking I/O model that makes it lightweight and