Documentation/admin-guide: kernel-parameters: fix "io7" parameter description

Fix punctuation and capitalization for the "io7" boot parameter.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20200918054751.6538-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>