docs: kernel-doc: typo "if ... if" -> "if ... is"

"If no *function* if specified" should instead be
"If no *function* is specified".

Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>