doc(config): change set's name

[Kocal]

The PSR12 set exists (https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/3.0/src/RuleSet/Sets/PSR12Set.php), but here we say:

The example below will add two rules to the default list of PSR2 set rules.

Which is wrong.
I've fixed the set to PSR2, but maybe you want to use PSR12 instead.

Let me know!

[julienfalque]

maybe you want to use PSR12 instead.

Yeah I would rather keep PSR-12 which supersedes PSR-2 :)

[Kocal]

Done :)

[coveralls]

Coverage remained the same at 92.164% when pulling f888d66 on Kocal:patch-1 into 1dc0f21 on FriendsOfPHP:3.0.

[keradus]

Thank you @Kocal.