Clean up grammar from rails#16752 [ci skip]

Author: Zachary Scott

guides/source/layouts_and_rendering.md

@@ -248,8 +248,8 @@ service requests that are expecting something other than proper HTML.
 
 NOTE: By default, if you use the `:plain` option, the text is rendered without
 using the current layout. If you want Rails to put the text into the current
-layout, you need to add the `layout: true` option. Make sure to add a layout
-file with extension `.txt.erb`.
+layout, you need to add the `layout: true` option and use the `.txt.erb`
+extension for the layout file.
 
 #### Rendering HTML