minor #5710 Fix grammar mistake in security.rst (zatikbalazs)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #5710).

Discussion
----------

Fix grammar mistake in security.rst

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
-------

e8a50b5 Fix grammar mistake in security.rst

Author: xabbuh

book/security.rst

@@ -507,7 +507,7 @@ else, you'll want to encode their passwords. The best algorithm to use is
 
 .. include:: /cookbook/security/_ircmaxwell_password-compat.rst.inc
 
-Of course, your user's passwords now need to be encoded with this exact algorithm.
+Of course, your users' passwords now need to be encoded with this exact algorithm.
 For hardcoded users, you can use an `online tool`_, which will give you something
 like this: