Fix conjugation (#816)

Author: arnaudlimbourg

docs/install.rst

@@ -104,5 +104,5 @@ Raven expects the browser to provide `window.JSON` and
 `standards mode
 <http://msdn.microsoft.com/en-us/library/cc288325(VS.85).aspx>`_.  You can
 also use `json2.js <https://github.com/douglascrockford/JSON-js>`_ to
-provide the JSON implementation in browsers/modes which doesn't support
-native JSON
+provide the JSON implementation in browsers/modes which don't support
+native JSON.