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.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -940,7 +940,7 @@
 			Arch Perfmon v4 (Skylake and newer).
 
 	disable_ddw	[PPC/PSERIES]
-			Disable Dynamic DMA Window support. Use this if
+			Disable Dynamic DMA Window support. Use this
 			to workaround buggy firmware.
 
 	disable_ipv6=	[IPV6]