Update Docs on Logging

[CHH]

The documentation says

A log is auto-injected into your task Job class without having to enable it. To set the logging level, just add
something like this to your grails-app/conf/Config.groovy log4j configuration.

debug 'grails.app.jobs'

This doesn't work and this should be:

debug 'grails.app.task'

as suggested on the Grails Mailinglist and tested with Grails 1.3.7 / grails-quartz 0.4.2