docs: mm/gup: Minor documentation update

Now there are 5 cases. Updated the same.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Link: https://lore.kernel.org/r/1592422023-7401-1-git-send-email-jrdr.linux@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/pin_user_pages.rst

@@ -33,7 +33,7 @@ all combinations of get*(), pin*(), FOLL_LONGTERM, and more. Also, the
 pin_user_pages*() APIs are clearly distinct from the get_user_pages*() APIs, so
 that's a natural dividing line, and a good point to make separate wrapper calls.
 In other words, use pin_user_pages*() for DMA-pinned pages, and
-get_user_pages*() for other cases. There are four cases described later on in
+get_user_pages*() for other cases. There are five cases described later on in
 this document, to further clarify that concept.
 
 FOLL_PIN and FOLL_GET are mutually exclusive for a given gup call. However,