Bug 10186: Fix a typo in subscription-add.tt : 2/years should be 2/year

Bug 10186: Fix a typo in subscription-add.tt : 2/years should be 2/year

In subscription-add.tt dropdown list for periodiciy, the description for 9 value should be "2/year" (2 issues each year)
At present, if the option value is selected, it is "2/years".
This patch supresses the "s" after "year".

To test :
- in a subscription, change periodicity to "2/year"
- save the subscription
- reopen it and check the value is labelled "2/year"

Signed-off-by: David Cook <dcook@prosentient.com.au>
Trivial change. Works as described.
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>