Fixed typo

Author: richardmiller-zz

contributing/documentation/overview.rst

@@ -26,7 +26,7 @@ then clone your fork:
     $ git clone git://github.com/YOURUSERNAME/symfony-docs.git
 
 Unless you're documenting a feature that's new to Symfony 2.1, you changes
-should be based on the 2.0 branch instead not the master branch. To do this
+should be based on the 2.0 branch instead of the master branch. To do this
 checkout the 2.0 branch before the next step:
 
 .. code-block:: bash