Fix typo in doc/administration/gitaly/index.md

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6900.
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/administration/gitaly/index.md

@@ -149,7 +149,7 @@ listen_addr = '0.0.0.0:8075'
 [auth]
 token = 'abc123secret'
 
-[[storage]
+[[storage]]
 name = 'default'
 path = '/mnt/gitlab/default/repositories'