Wording nit in README.md (#894)

Summary:
Change "If advanced debugging functionality is required" to "If advanced debugging functionality is required, use:" for consistency in README.md and to be more reader friendly.
Pull Request resolved: https://github.com/facebook/folly/pull/894

Reviewed By: shixiao

Differential Revision: D9094182

Pulled By: yfeldblum

fbshipit-source-id: 147d436aaee291104b67eab1e549554696740907

README.md

@@ -130,7 +130,7 @@ sudo apt-get install \
     pkg-config
 ```
 
-If advanced debugging functionality is required
+If advanced debugging functionality is required, use:
 
 ```
 sudo apt-get install \