Bug 13051: And remember, usernames are not case sensitive

Bug 13051: And remember, usernames are not case sensitive

opac-auth contains the following statement:
You entered an incorrect username or password. Please try again! And
remember, usernames and passwords are case sensitive.

Obviously, this is not completely true. The username can be entered in
lowercase, uppercase, any mixed case.

This patch simply adjusts this string.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>