Corrected postBuffer docs

What does this MR do and why?

Applying this snippet to the gitlab.rb configuration and running gitlab-ctl reconfigure results in an error when importing a repository from an HTTPS URL. The error states that > 524288000 is an incorrect value.

omnibus_gitconfig['system'] = {
  # Set the http.postBuffer size, in bytes
  "http" => ["postBuffer => 524288000"]
}

The notation is incorrect for usage in this array, => would be used inside a hash.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.