docs/misc: livepatch: Escape backslash

docs/misc: livepatch: Escape backslash

pandoc is currently failing to generate the pdf with the following
error:
! Undefined control sequence.
l.1048   metadata string format is: key=value\0

In this case, we want to print \0 so we need to backslash-escape the
first character.

Interestingly pandoc will not complain when creating html and will just
ignore \0 completely.

Fixes: 5083e0ff93 ("livepatch: Add metadata runtime retrieval mechanism")
Signed-off-by: Julien Grall <julien@xen.org>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Pawel Wieczorkiewicz <wipawel@amazon.de>