Fixing typo in Readme

Summary: Closes https://github.com/facebook/folly/pull/403

Reviewed By: yfeldblum

Differential Revision: D3320043

Pulled By: elliottneilclark

fbshipit-source-id: 5d21421216180fb601015a40a3de79e3dd2a162e

README.md

@@ -109,7 +109,7 @@ sudo apt-get install \
     libssl-dev
 ```
 
-If advanced dubugging functionality for tests are required
+If advanced debugging functionality for tests are required
 
 ```
 sudo apt-get install \