filemap: fix htmldoc warning for mapping_align_index()

Stephen reported that there is a kernel build warning due to a missing
description of a parameter in mapping_align_index().

Add the missing index parameter in the comment description.
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Link: https://lore.kernel.org/r/20240827084206.106347-2-kernel@pankajraghav.com
Fixes: ab95d23b ("filemap: allocate mapping_min_order folios in the page cache")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/pagemap.h

@@ -470,6 +470,7 @@ mapping_min_folio_nrpages(struct address_space *mapping)
 /**
  * mapping_align_index() - Align index for this mapping.
  * @mapping: The address_space.
+ * @index: The page index.
  *
  * The index of a folio must be naturally aligned.  If you are adding a
  * new folio to the page cache and need to know what index to give it,