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>

Documentation/doc-guide/kernel-doc.rst

@@ -490,7 +490,7 @@ doc: *title*
 
 functions: *[ function ...]*
   Include documentation for each *function* in *source*.
-  If no *function* if specified, the documentation for all functions
+  If no *function* is specified, the documentation for all functions
   and types in the *source* will be included.
 
   Examples::