[ci skip] Fix the grammar

This is discussed on https://github.com/rails/rails/pull/21334

guides/source/api_documentation_guidelines.md

@@ -87,7 +87,7 @@ Please use American English (*color*, *center*, *modularize*, etc). See [a list
 Comma
 -------
 
-Please use Oxford comma (*red, white, and blue* style). See [the detail of Oxford comma](http://en.wikipedia.org/wiki/Serial_comma).
+Please use the Oxford comma (*red, white, and blue* style). See [the detail of Oxford comma](http://en.wikipedia.org/wiki/Serial_comma).
 
 Example Code
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