rev-list-options.txt: fix rendering of bonus paragraph

In git-log(1) -- but not in git-shortlog(1) or git-rev-list(1) -- we
include a bonus paragraph in the description of `--first-parent`. But
we forgot to add a lone "+" for a list continuation, and we shouldn't
be indenting this second paragraph. As a result, we get a different
indentation and the `backticks` render literally.

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

Author: Martin Ågren

Documentation/rev-list-options.txt

@@ -129,10 +129,10 @@ parents) and `--max-parents=-1` (negative numbers denote no upper limit).
 	adjusting to updated upstream from time to time, and
 	this option allows you to ignore the individual commits
 	brought in to your history by such a merge.
-
 ifdef::git-log[]
-	This option also changes default diff format for merge commits
-	to `first-parent`, see `--diff-merges=first-parent` for details.
++
+This option also changes default diff format for merge commits
+to `first-parent`, see `--diff-merges=first-parent` for details.
 endif::git-log[]
 
 --not::