Updated CONTRIBUTING.md: Mention of "facebook.com" was not a link

Updated the mention of "facebook.com" on line 3 of the CONTRIBUTING.md file to link to Facebook. Previously, this mention was just in plain text.

Changed from `facebook.com` to `[facebook.com](https://facebook.com)`

No other content has been changed. Content has not been removed from this file in any way.

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Contributing to React
 
-React is one of Facebook's first open source projects that is both under very active development and is also being used to ship code to everybody on facebook.com. We're still working out the kinks to make contributing to this project as easy and transparent as possible, but we're not quite there yet. Hopefully this document makes the process for contributing clear and preempts some questions you may have.
+React is one of Facebook's first open source projects that is both under very active development and is also being used to ship code to everybody on [facebook.com](https://facebook.com). We're still working out the kinks to make contributing to this project as easy and transparent as possible, but we're not quite there yet. Hopefully this document makes the process for contributing clear and preempts some questions you may have.
 
 ## Our Development Process