grammar fix in comment

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/mm/highmem.c b/mm/highmem.c
index 9c1e627..bed8a8b 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -220,7 +220,7 @@
  * @page: &struct page to pin
  *
  * Returns the page's current virtual memory address, or NULL if no mapping
- * exists.  When and only when a non null address is returned then a
+ * exists.  If and only if a non null address is returned then a
  * matching call to kunmap_high() is necessary.
  *
  * This can be called from any context.