treewide: Fix typo in Documentation/DocBook

This patch fix spelling typo in Documentation/DocBook.
It is because .html and .xml files are generated by make htmldocs,
I have to fix a typo within the source files.

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/include/linux/skbuff.h b/include/linux/skbuff.h
index 215b5ea..cde8425 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1951,7 +1951,7 @@
 }
 
 /**
- * skb_frag_page - retrieve the page refered to by a paged fragment
+ * skb_frag_page - retrieve the page referred to by a paged fragment
  * @frag: the paged fragment
  *
  * Returns the &struct page associated with @frag.