Fix broken link [ci-skip]

Follow-up to rails#45752.

Author: jonathanhefner

guides/source/testing.md

@@ -1898,7 +1898,7 @@ class UsersTest < ActionDispatch::SystemTestCase
 end
 ```
 
-NOTE: The `assert_emails` method is not tied to a particular deliver method and will work with emails delivered with either the `deliver_now` or `deliver_later` method. If we explicitly want to assert that the email has been enqueued we can use the `assert_enqueued_email_with` ([examples above](#testing_enqueued_emails)) or `assert_enqueued_emails` methods. More information can be found in the  [documentation here](https://api.rubyonrails.org/classes/ActionMailer/TestHelper.html).
+NOTE: The `assert_emails` method is not tied to a particular deliver method and will work with emails delivered with either the `deliver_now` or `deliver_later` method. If we explicitly want to assert that the email has been enqueued we can use the `assert_enqueued_email_with` ([examples above](#testing-enqueued-emails)) or `assert_enqueued_emails` methods. More information can be found in the [documentation here](https://api.rubyonrails.org/classes/ActionMailer/TestHelper.html).
 
 Testing Jobs
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