doc/ci: include local: remove superfluous ":"

such extra colon syntax is not documented. so assume it's a typo.

includes:
  - local: : /templates/docker-build.yml
  - local: : /templates/docker-testing.yml

the change was introduced in https://gitlab.com/gitlab-org/gitlab-ce/commit/c78861bc4268049777bd279f5de5981ec4e78019

cc @ayufan @eread