Fix GitHub/GitLab brand capitalization

[sqs]

This change ensures consistent capitalization of the GitHub and GitLab brand names in Markdown documentation.

Created by Sourcegraph batch change sqs/capitalize-brands-correctly.

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Sourcegraph.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email [email protected]
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails