[Query Workbench] - Don't add extra ` when auto-completing

Minor user friendliness issue...

If I'm start typing a bucket name and know that I need to use backticks, I subconciously enter the first backtick followed by a few letters of the bucket name and hit tab.  However, when it autocompletes, there is an extra backtick at the beginning and the query will fail.

i.e, type: "select * from `tra" <tab>

get: "select * from ``travel-sample`" and it fails until you remove the second backtick at the beginning.