minor #3733 [book] [validation] fixed minor typos (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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[book] [validation] fixed minor typos

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | -

Commits
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693ba24 [book] [validation] fixed minor typos

book/validation.rst

@@ -974,8 +974,8 @@ that group are valid, the second group, ``Strict``, will be validated.
     constraints that do not belong to any group.
 
     This means that you have to use the ``{ClassName}`` (e.g. ``User``) group
-    when specifing a group sequence. When using ``Default``, you get an
-    infinite recursion (as the ``Default`` groups references the group
+    when specifying a group sequence. When using ``Default``, you get an
+    infinite recursion (as the ``Default`` group references the group
     sequence, which will contain the ``Default`` group which references the
     same group sequence, ...).