Documentation: PCI: Fix typos in msi-howto.rst

Documentation: PCI: Fix typos in msi-howto.rst

Fix subject-verb agreement for "has a requirements" as well as
"neither...or" conjunction mistake. And convert "Message Signalled
Interrupts" to "Message Signaled Interrupts" to match the PCIe spec.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://patch.msgid.link/1766713528-173281-1-git-send-email-shawn.lin@rock-chips.com