config.txt: follow camelCase naming

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

diff --git a/Documentation/config.txt b/Documentation/config.txt
index eb66a11..35e024d 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -3291,7 +3291,7 @@
 	Seconds wait before reconnecting to smtp server.
 	See also the `--relogin-delay` option of linkgit:git-send-email[1].
 
-showbranch.default::
+showBranch.default::
 	The default set of branches for linkgit:git-show-branch[1].
 	See linkgit:git-show-branch[1].