minor #4775 Corrected validation information on inheritance (peterrehm)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #4775).

Discussion
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Corrected validation information on inheritance

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -
| Related | #4454

Minor fix as per Discussion 09f9d3d#commitcomment-9150852

Commits
-------

230c4fc Corrected validation information on inheritance

book/validation.rst

@@ -844,8 +844,8 @@ the string ``Default``.
     If you have inheritance (e.g. ``User extends BaseUser``) and you validate
     with the class name of the subclass (i.e. ``User``), then all constraints
     in the ``User`` and ``BaseUser`` will be validated. However, if you validate
-    using the base class (i.e. ``BaseUser``), then only the constraints in
-    the ``BaseUser`` group will be validated.
+    using the base class (i.e. ``BaseUser``), then only the default constraints in
+    the ``BaseUser`` class will be validated.
 
 To tell the validator to use a specific group, pass one or more group names
 as the second argument to the ``validate()`` method::