rebase: fix documentation formatting

Last sections are squashed into non-formatted block after adding
"REBASING MERGES".
To reproduce the error see bottom of page:
https://git-scm.com/docs/git-rebase

Signed-off-by: Vladimir Parfinenko <vparfinenko@excelsior-usa.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -804,7 +804,7 @@ The ripple effect of a "hard case" recovery is especially bad:
 case" recovery too!
 
 REBASING MERGES
------------------
+---------------
 
 The interactive rebase command was originally designed to handle
 individual patch series. As such, it makes sense to exclude merge