Bug 27470: Improved link text for shibboleth login

Bug 27470: Improved link text for shibboleth login

TEST PLAN:
1) Enable shibboleth by adding shibboleth to koha-conf.xml, you can do
   this by following

   https://wiki.koha-community.org/wiki/Shibboleth_Configuration#Using_AD_FS_Metadata

2) If you are logged in Koha, log out, on the login screen the text
   at the top should say the following, "Log in using a Shibboleth
   account"

Sponsored-by: Catalyst IT
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>