py: fix syntax error

Thanks to @adamreyher for spotting.

Author: andreastt

py/selenium/webdriver/firefox/webdriver.py

@@ -63,7 +63,7 @@ def __init__(self, firefox_profile=None, firefox_binary=None, timeout=30,
                 remote_server_addr=self.service.service_url)
             RemoteWebDriver.__init__(
                 self,
-                command_executor=executor
+                command_executor=executor,
                 desired_capabilities=capabilities,
                 keep_alive=True)