docs: MERGE_AUTOSTASH is not that special

A handful of manual pages called MERGE_AUTOSTASH a "special ref",
but there is nothing special about it.  It merely is yet another
pseudoref.

Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

Documentation/merge-options.txt

@@ -191,7 +191,7 @@ endif::git-pull[]
 --autostash::
 --no-autostash::
 	Automatically create a temporary stash entry before the operation
-	begins, record it in the special ref `MERGE_AUTOSTASH`
+	begins, record it in the ref `MERGE_AUTOSTASH`
 	and apply it after the operation ends.  This means
 	that you can run the operation on a dirty worktree.  However, use
 	with care: the final stash application after a successful