user-manual.txt: prefer 'merge --abort' over 'reset --hard'

Since the operation in progress is merge, stick to the 'git merge'
variant of aborting. 'git reset --hard' does not really tell you about
aborting the merge by just looking, longer to type, and even though I
know by heart what --hard do, I still dislike it when I need to consider
whether --hard, --mixed or --soft.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -1408,7 +1408,7 @@ If you get stuck and decide to just give up and throw the whole mess
 away, you can always return to the pre-merge state with
 
 -------------------------------------------------
-$ git reset --hard HEAD
+$ git merge --abort
 -------------------------------------------------
 
 Or, if you've already committed the merge that you want to throw away,