Fix expression editor documentation link

HOW TO REPRODUCE

• Open the MVEL expression editor in any Studio screen (e.g. automation chain parameter, workflow transition condition)
• Try to follow the link labeled online help

=> You are taken to your Studio project again in another tab

TODO
Replace current link href property

<a href="" target="_blank">online help</a>

with https://doc.nuxeo.com/nxdoc/use-of-mvel-in-automation-chains/