Incorrect command flags in documentation on page "Using Docker images"

Summary

The command line given in the "Register Docker Runner" section contains a flag that is not supported.

Steps to reproduce

• Navigate to https://gitlab.com/help/ci/docker/using_docker_images.md
• Run the command given in "Register Docker Runner"

What is the current bug behavior?

The message Incorrect Usage: flag provided but not defined: -docker-postgres is given, followed by a dump of command flags.

What is the expected correct behavior?

The config.toml file is created with a working Docker configuration.

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's very hard to read otherwise.)

Output of checks

(not applicable)

Results of GitLab environment info

gitlab-runner-version:

Version:      10.7.0
Git revision: 7c273476
Git branch:   10-7-stable
GO version:   go1.8.7
Built:        2018-04-22T13:43:35+00:00
OS/Arch:      linux/amd64

Results of GitLab application Check

(not applicable)

Possible fixes

The flag appears to have been changed to --docker-services postgres:latest. (A similar fix would be needed for the mysql flag given after the postgres one…)