Documentation/admin-guide: kernel-parameters: fix "disable_ddw" wording

Drop and extraneous word (if) in a sentence.

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/20200918054803.6588-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>