geckodriver: readme: correct english

Source-Repo: https://github.com/mozilla/geckodriver
Source-Revision: 2dbb490830f3bf32ffde9e86fce7f69c0b03629c

--HG--
extra : rebase_source : dc5856198ddde4ec05ce2082f1e878caadaa077d

testing/geckodriver/README.md

@@ -19,7 +19,7 @@ on [supported platforms](#supported-firefoxen).
 ## Supported Firefoxen
 
 Marionette and geckodriver are not yet feature complete.
-This means it does not yet offer full conformance
+This means that they do not yet offer full conformance
 with the [WebDriver standard](https://w3c.github.io/webdriver/webdriver-spec.html)
 or complete compatibility with [Selenium](http://www.seleniumhq.org/).