minor #19392 [Validator] update default message (xabbuh)

This PR was merged into the 7.1 branch.

Discussion
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[Validator] update default message

related to symfony/symfony#53444

Commits
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5dcaa08 update default message

Author: javiereguiluz

reference/constraints/MacAddress.rst

@@ -87,7 +87,7 @@ Options
 ``message``
 ~~~~~~~~~~~
 
-**type**: ``string`` **default**: ``This is not a valid MAC address.``
+**type**: ``string`` **default**: ``This value is not a valid MAC address.``
 
 This is the message that will be shown if the value is not a valid MAC address.