[Validator] Missing code example in the IdenticalTo constraint

[xElysioN]

Hi,
In the IdenticalTo constraint, there is missing codes in YAML, XML and PHP for the firstname constraint in the Basic Usage, but ok for annotations.

CF : https://symfony.com/doc/3.4/reference/constraints/IdenticalTo.html#basic-usage

[javiereguiluz]

Fixed by #10830.