fix filemap.c kernel-doc

Fix kernel-doc warning:
Warning(linux-2.6.23-rc1-mm1//mm/filemap.c:864): No description found for parameter 'ra'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -843,7 +843,7 @@ static void shrink_readahead_size_eio(struct file *filp,
 /**
  * do_generic_mapping_read - generic file read routine
  * @mapping:	address_space to be read
- * @_ra:	file's readahead state
+ * @ra:		file's readahead state
  * @filp:	the file to read
  * @ppos:	current file position
  * @desc:	read_descriptor