Bug 32665: Fix typo in template condition

Bug 32665: Fix typo in template condition

This patch fixes a typo in a template condition so that
the about.pl warning about RequireChoosingExistingAuthority is
shown when it's the only warning to display

Test plan:
0. Don't apply the patch yet
1. Go to http://localhost:8081/cgi-bin/koha/about.pl
2. Note that "No warnings" is shown
3. Apply the patch
4. Go to http://localhost:8081/cgi-bin/koha/about.pl
5. Note that there is a heading for "Warnings regarding the
system configuration" and a specific warning about the
"RequireChoosingExistingAuthority" system preference

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>