Documentation: Docbook: Fix generated DocBook/kernel-api.xml

This patch fix spelling typo found in DocBook/kernel-api.xml.
It is because the file is generated from the source comments,
I have to fix the comments in source codes.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/lib/idr.c b/lib/idr.c
index 39158ab..460abfd 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -625,7 +625,7 @@
  * idr_destroy().
  *
  * A typical clean-up sequence for objects stored in an idr tree will use
- * idr_for_each() to free all objects, if necessay, then idr_destroy() to
+ * idr_for_each() to free all objects, if necessary, then idr_destroy() to
  * free up the id mappings and cached idr_layers.
  */
 void idr_destroy(struct idr *idp)