Documentation/config: typofix

Each heading of enumerated list should end with double-colon, not single.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -860,7 +860,7 @@ pack.indexVersion::
 	whenever the corresponding pack is larger than 2 GB.  Otherwise
 	the default is 1.
 
-pack.packSizeLimit:
+pack.packSizeLimit::
 	The default maximum size of a pack.  This setting only affects
 	packing to a file, i.e. the git:// protocol is unaffected.  It
 	can be overridden by the `\--max-pack-size` option of