config.txt: fix formatting of submodule.alternateErrorStrategy section

Add missing `::` after the title.

Signed-off-by: David Pursehouse <dpursehouse@collab.net>
Acked-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2844,7 +2844,7 @@ submodule.alternateLocation::
 	value is set to `superproject` the submodule to be cloned computes
 	its alternates location relative to the superprojects alternate.
 
-submodule.alternateErrorStrategy
+submodule.alternateErrorStrategy::
 	Specifies how to treat errors with the alternates for a submodule
 	as computed via `submodule.alternateLocation`. Possible values are
 	`ignore`, `info`, `die`. Default is `die`.