Add 'in' to complete thought

Simple grammar fix.

CONTRIBUTING.md

@@ -17,7 +17,7 @@ We only accept issues that are bug reports or feature requests. Bugs must be iso
 
 ## Pull requests
 
-- CSS changes must be done in `.less` files first, never just the compiled `.css` files
+- CSS changes must be done in `.less` files first, never just in the compiled `.css` files
 - If modifying the `.less` files, always recompile and commit the compiled files `bootstrap.css` and `bootstrap.min.css`
 - Try not to pollute your pull request with unintended changes--keep them simple and small
 - Try to share which browsers your code has been tested in before submitting a pull request