Minor docs fix.

Author: Paul McMillan

docs/quickstart.rst

@@ -744,7 +744,7 @@ sessions work::
 
     @app.route('/logout')
     def logout():
-        # remove the username from the session if its there
+        # remove the username from the session if it's there
         session.pop('username', None)
         return redirect(url_for('index'))