minor #8912 add validator dependency (ebuildy)

This PR was merged into the 4.0 branch.

Discussion
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add validator dependency

Commits
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e284451 add validator dependency

Author: xabbuh

doctrine/registration_form.rst

@@ -14,7 +14,7 @@ First, make sure you have all the dependencies you need installed:
 
 .. code-block:: terminal
 
-    $ composer require doctrine form security
+    $ composer require doctrine form security validator
 
 .. tip::