Bug 15407: [QA Follow-up] Confusing cathash removed

Bug 15407: [QA Follow-up] Confusing cathash removed

One of the patches removed %cathash. Some kind of silly remainder
was left in a string. Just used as a text field to explain the
filters used on borrowers_stats.pl. The filter name cathash is not
really easy and also was confusing in relation to the variable.

So just a string patch, replacing it with Patron category.

NOTE: The output of borrowers_stats.pl with the filters listed, is
not one of the most beautiful Koha screens.
Furthermore, translatibility of these filters is still an issue to
be handled on another report.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>