Bug 27740: (follow-up) changed "User summary" css

Bug 27740: (follow-up) changed "User summary" css

Changed user summary CSS to style H2 not H3 heading

To test:
1) Go to OPAC homepage
2) Login as a user with at least one loan
3) See that the "User summary" heading is not styled
4) Apply patch
5) Run yarn build --view OPAC
6) Confirm the heading is now styled with grey background

Signed-off-by: Wainui Witika-Park <wainuiwitikapark@catalyst.net.nz>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>