Merge pull request #19312 from mariusbutuc/fix-mailer-testing-typo

Fix typo in the Testing Your Mailers docs

guides/source/testing.md

@@ -1083,7 +1083,7 @@ Testing mailer classes requires some specific tools to do a thorough job.
 
 ### Keeping the Postman in Check
 
-Your mailer classes - like every other part of your Rails application - should be tested to ensure that it is working as expected.
+Your mailer classes - like every other part of your Rails application - should be tested to ensure that they are working as expected.
 
 The goals of testing your mailer classes are to ensure that: