Bug 7843: (follow up) Improve RSS feed language

Bug 7843: (follow up) Improve RSS feed language

The RSS button used to read ' RSS for the librarys general newsfeed.'
This included one grammatical error and I think included more words
than most average people would read.  This patch simplifies it to say
'RSS for Library News.'

To test:
* Apply all patches
* Review RSS feed button's text on main page under news

http://bugs.koha-community.org/show_bug.cgi?id=7843

Signed-off-by: Pierre <tredok.pierre@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>