Fix link in guides/action_mailer_basics

The link to "testing guide" used an invalid fragment identifier.

Author: drjayvee

guides/source/action_mailer_basics.md

@@ -1013,7 +1013,7 @@ Previewing and Testing Mailers
 ------------------------------
 
 You can find detailed instructions on how to test your mailers in the [testing
-guide](testing.html#testing-your-mailers).
+guide](testing.html#testing-mailers).
 
 ### Previewing Emails