minor #14926 Change deprecated psr 2 to psr 12 from doc (woprrr)

This PR was submitted for the 5.2 branch but it was merged into the 4.4 branch instead.

Discussion
----------

Change deprecated psr 2 to psr 12 from doc

PSR 2 => PSR 12

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `5.x` for features of unreleased versions).

-->

Commits
-------

4539584 Change deprecated psr 2 to psr 12 from doc

Author: javiereguiluz

contributing/code/standards.rst

@@ -5,7 +5,7 @@ Symfony code is contributed by thousands of developers around the world. To make
 every piece of code look and feel familiar, Symfony defines some coding standards
 that all contributions must follow.
 
-These Symfony coding standards are based on the `PSR-1`_, `PSR-2`_ and `PSR-4`_
+These Symfony coding standards are based on the `PSR-1`_, `PSR-12`_ and `PSR-4`_
 standards, so you may already know most of them.
 
 Making your Code Follow the Coding Standards