Update monolog.rst

Change method 'err' to 'error' in the example because the 'err' method is deprecated and to be removed. (As specified in this file : vendor/symfony/symfony/src/Symfony/Bridge/Monolog/Logger.php on line 42 : @deprecated since 2.2, to be removed in 3.0. Use error() which is PSR-3 compatible.)
"The label option" wasn't translated
Merge pull request #502 from sylvaincombes/patch-1

Update monolog.rst
Merge pull request #505 from jdeniau/patch-1

"The label option" wasn't translated
Merge branch 'master' into 2.4
Merge pull request #506 from symfony-fr/2.4

Merging 2.4