doc: update the order of the syntax `git merge --continue`

The syntax "git merge <message> HEAD <commit>" has been removed. The
order of the syntax should also be updated.

Signed-off-by: Meng-Sung Wu <mengsungwu@fortunewhite.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -57,7 +57,7 @@ reconstruct the original (pre-merge) changes. Therefore:
 discouraged: while possible, it may leave you in a state that is hard to
 back out of in the case of a conflict.
 
-The fourth syntax ("`git merge --continue`") can only be run after the
+The third syntax ("`git merge --continue`") can only be run after the
 merge has resulted in conflicts.
 
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