Fix custom EL expression sample on workflow availability tag

Steps to reproduce

In Nuxeo studio:
Go to Workflow > Workflow definition
Create a new workflow
Go to the Availability tab
Click on the help icon next to the Custom EL expression field

The problem

The help tooltip indicates the following example:
Sample:

#{currentDocument.dc.title == 'myTitle'}

but this expression is not working.

What is working

The following expression works:

#{currentDocument.getPropertyValue('dc:title') == 'myTitle'}