[SecurityBundle] Wrong note about password encoders

[chalasr]

Good first issue:
In https://symfony.com/doc/3.4/reference/configuration/security.html#encoders, the first config block contains:

# PBKDF2 encoder using SHA512 hashing with default options
AppBundle\Entity\User: 'sha512'

Actually such config does not wire a PBKDF2 encoder but a MessageDigestPasswordEncoder which uses the hash() function.
(you'd need AppBundle\Entity\User: pbkdf2 for a PBKDF2 encoder with sha512 hash algo).