minor #6088 Update validation.rst (syedi)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes #6088).

Discussion
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Update validation.rst

| Q               | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 3.0
| Fixed tickets | N/A

place 'IsTrue' instead of 'True' in line 957

Commits
-------

5e4e5a7 Update validation.rst

Author: xabbuh

book/validation.rst

@@ -945,7 +945,7 @@ username and the password are different only if all other validation passes
                 - Strict
             getters:
                 passwordLegal:
-                    - 'True':
+                    - 'IsTrue':
                         message: 'The password cannot match your username'
                         groups: [Strict]
             properties: