user-manual: reset to ORIG_HEAD not HEAD to undo merge

As Linus pointed out recently on the mailing list,

	git reset --hard HEAD^

doesn't undo a merge in the case where the merge did a fast-forward.  So
the rcommendation here is a little dangerous.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/user-manual.txt

@@ -1255,7 +1255,7 @@ $ git reset --hard HEAD
 Or, if you've already commited the merge that you want to throw away,
 
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-$ git reset --hard HEAD^
+$ git reset --hard ORIG_HEAD
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 However, this last command can be dangerous in some cases--never