docs/man: xl.pod.1: Fix build issue after commit 7471879

docs/man: xl.pod.1: Fix build issue after commit 7471879

The commit 7471879 "tools: CMDs and APIs for Cache Monitoring Technology"
has introduced a new section but forgot to add '=back'. This will result
a build issue with newer podman:

man/xl.pod.1 around line 1443: You forgot a '=back' before '=head1'

or creating a section "POD ERRORS" with older one.

At the same time, capitalize the title of the section to match the coding
style used within the files.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>