PSR-1 incorrectly cited as recommending CamelCase properties

In the Serializer documentation here it says

However, PSR-1 specifies that the preferred style for PHP properties and methods is CamelCase.

Regarding properties this seem to not match PSR-1, though, which says

4.2. Properties
This guide intentionally avoids any recommendation regarding the use of $StudlyCaps, $camelCase, or $under_score property names.

[javiereguiluz]

Fixed by #8995.