LDAP docs - Consistent indent for example config, one click for the rake task

What does this MR do?

1️⃣ The indentation for example LDAP config is really hard to read, and so it's easy to end up with reconfigure failing because there's a missing }

Looking at the example at a high level:

gitlab_rails['ldap_servers'] = {
'main' => {
  'tls_options' => {
  },
  'attributes' => {
  },
  }
}

'tls_options' and 'attributes' are indented the same amount as their closing }, but main is not, and it's the same in the example with multiple LDAP sources.

My reading was that 'lowercase_usernames' => false, and the # EE Only setting are global gitlab_rails['ldap_servers'] settings, but they're not - they're part of main.

2️⃣ After configuring LDAP, to test the configuration, use the LDAP check Rake task. links to a paragraph that says This task is also executed as part of the gitlab:check task, but can run independently. See LDAP Rake Tasks - LDAP Check for details. I'm removing that extra click.

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