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>

mm/highmem.c

@@ -220,7 +220,7 @@ EXPORT_SYMBOL(kmap_high);
  * @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.