Bug 9394: (follow-up) fix query column alias

Bug 9394: (follow-up) fix query column alias

A change-and-replace went a tick too far.  This patch
adjusts the column alias in the query run in MergeHolds()
to reflect that the value being returned is the number of
hold requests, not an ID.

To test:

[1] This patch should have no visible changes to behavior.  To
    verify, pick to bib records that have hold requests on them,
    then merge them together.  Verify that the merged bib
    contains sll of the hold requests on it.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>