config doc: add missing list separator for checkout.optimizeNewBranch

The documentation added in fa655d8411 ("checkout: optimize "git
checkout -b <new_branch>"", 2018-08-16) didn't add the double-colon
needed for the labeled list separator, as a result the added
documentation all got squashed into one paragraph. Fix that by adding
the list separator.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/config.txt b/Documentation/config.txt
index 1a2deea..139f552 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -1101,7 +1101,7 @@
 	browse HTML help (see `-w` option in linkgit:git-help[1]) or a
 	working repository in gitweb (see linkgit:git-instaweb[1]).
 
-checkout.optimizeNewBranch
+checkout.optimizeNewBranch::
 	Optimizes the performance of "git checkout -b <new_branch>" when
 	using sparse-checkout.  When set to true, git will not update the
 	repo based on the current sparse-checkout settings.  This means it