Fix quote mark in postBuffer settings

What does this MR do and why?

This merge request fixes a small error in the docs where a end quote needs to be moved to the end of the statement. Copy paste of the snippet results in an unusable server. Chef is evaluating the example into json instead of a normal ini type statement.

Screenshots or screen recordings

It's not really necessary

How to set up and validate locally

Copy and paste the example from the docs into /etc/gitlab/gitlab.rb and reconfigure / then restart. You will see errors thrown by gitlab-rails.

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.