Bug 18939: Acquisition statistics - Fix wrong template variable name for collection...

Bug 18939: Acquisition statistics - Fix wrong template variable name for collection filter

The template variable was wrong, so the filter never submitted the
collection code for the query.

Before:
<option value="">Fiction</option>
<option value="">Non-fiction</option>

After:
<option value="FIC">Fiction</option>
<option value="NFIC">Non-fiction</option>

To test:
- Go to Reports > Statistics wizards > Acquisitions
- Pick a row and column
- Compare results with and without collection filter
- Verify the table is always the same
- Apply patch
- Repeat comparison, verify the filter works now

https://bugs.koha-community.org/show_bug.cgi?id=18393
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>