Update docs.md
Update docs.md

`$grav['logger']->` throws an exception. Proper way of logging is `$grav['log']->`.

Examples of it's use:
https://github.com/getgrav/grav/blob/34e50aab21b2144c0bb21e9ae93074ffa211877d/system/src/Grav/Common/Page/Pages.php#L282
https://github.com/getgrav/grav/blob/34e50aab21b2144c0bb21e9ae93074ffa211877d/system/src/Grav/Common/Page/Pages.php#L295
attiks/grav-learn,lahmizzar/grav-learn,nickanderson/grav-learn,miljan-aleksic/grav-learn,Quy/grav-learn,lahmizzar/grav-learn,getgrav/grav-learn,Quy/grav-learn,cord/grav-learn,simogeo/grav-learn,getgrav/grav-learn,Vivalldi/grav-learn,zikkuratvk/grav-learn,Collizo4sky/grav-learn,erictherobot/grav-learn,erictherobot/grav-learn,Vivalldi/grav-learn,nickanderson/grav-learn,zikkuratvk/grav-learn,lahmizzar/grav-learn,miljan-aleksic/grav-learn,attiks/grav-learn,simogeo/grav-learn,miljan-aleksic/grav-learn,getgrav/grav-learn,Collizo4sky/grav-learn,Vivalldi/grav-learn,erictherobot/grav-learn,attiks/grav-learn,Collizo4sky/grav-learn,cord/grav-learn,Quy/grav-learn,nickanderson/grav-learn,simogeo/grav-learn,getgrav/grav-learn,miljan-aleksic/grav-learn,Quy/grav-learn,cord/grav-learn,zikkuratvk/grav-learn