Docs: MSI-HOWTO: Insert `that'

... as per Randy Dunlap's wishes :-P
Message-Id: <20110717114023.2b4cce91.rdunlap@xenotime.net>

Signed-off-by: Michael Witten <mfwitten@gmail.com>
Acked-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>