doc: fix missing "::" in config list

The rebase.instructionFormat option is missing its "::" to
tell AsciiDoc that it's a list entry. As a result, the
option name gets lumped into the description in one big
paragraph.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2351,7 +2351,7 @@ rebase.missingCommitsCheck::
 	command in the todo-list.
 	Defaults to "ignore".
 
-rebase.instructionFormat
+rebase.instructionFormat::
 	A format string, as specified in linkgit:git-log[1], to be used for
 	the instruction list during an interactive rebase.  The format will automatically
 	have the long commit hash prepended to the format.