docs/uio: fix a grammar nitpick

This patch fixes a minor, incorrect piece of grammar in the UIO howto.

Signed-off-by: Will Korteland <will@korte.land>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>