mm: pagemap.h: fix two kernel-doc markups

Changeset a8cf7f27 ("mm: add find_lock_head") renamed the
index parameter, but forgot to update the kernel-doc markups
accordingly.

Fixes: a8cf7f27 ("mm: add find_lock_head")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/dce89b296a4f5f9f8f798d5e76b6736c14a916ac.1603791716.git.mchehab+huawei@kernel.org


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