docs: correct pod syntax

docs: correct pod syntax

Olaf reports:
[  146s] man/xl.pod.1 around line 1529: '=item' outside of any '=over'
[  146s] man/xl.pod.1 around line 1531: You forgot a '=back' before '=head1'
[  146s] POD document had syntax errors at /usr/bin/pod2text line 84.
[  146s] Makefile:167: recipe for target 'txt/man/xl.1.txt' failed
[  146s] make[1]: *** [txt/man/xl.1.txt] Error 255

Reported-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>