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.)
Merge pull request #502 from sylvaincombes/patch-1

Update monolog.rst