[ci skip] Corrected Grammatical Error

Author: Aditya Bhutani

guides/source/action_text_overview.md

@@ -126,7 +126,7 @@ end
 
 Action Text will sanitize and render rich content on your behalf.
 
-By default, the Action Text editor and content is styled by the Trix defaults.
+By default, the Action Text editor and content are styled by the Trix defaults.
 
 If you want to change these defaults, remove the `// require "actiontext.scss"`
 line from your `application.scss` to omit the [contents of that