Fixed a small typo in CONTRIBUTING.md

Summary:
## Motivation (required)

There was a small typo in CONTRIBUTING.md file, it's gone now :)

## Test Plan (required)

N/A
Closes facebookarchive#135

Reviewed By: amberroy

Differential Revision: D4937891

Pulled By: amberroy

fbshipit-source-id: 0df4b07

CONTRIBUTING.md

@@ -19,7 +19,7 @@ Before submitting a pull request, please make sure you have done the following:
 
 ## Copyright Notice for Files
 
-Copy and paste this to the tope of your new files:
+Copy and paste this to the top of your new files:
 
 ```
 /**