That script shouldn't be doing anything as there is not a field on the sc_req_item table named request_item. Since this is running on the sc_req_item table, your addQuery line should be

    gr.addQuery('request_item', current.sys_id);

You must have already discovered that, or it wouldn't be updating any sc_task records.  Just be aware that this will update the Details field on all Catalog Tasks that exist for this RITM.

Aside from all that, getJournalEntry(-1) translates to "retrieve all of the journal entries" when you really want just the most recent since this is running every time the comment changes, so use

		var comment = current.comments.getJournalEntry(1);