Fix typo in hook_civicrm_links

Created by: wmortada

The code that adds a hook to enable a link to be added to a block has a typo. It says 'shorcuts' rather than 'shortcuts'.

I have corrected this in the code - see https://github.com/civicrm/civicrm-core/pull/12913 - the documentation will need to be updated to reflect this change.