Docs: Change /token request to use HTTPS

What does this MR do?

I am working on a project that uses this Oauth2 web application flow. In my own development and testing, I have found that if I try the /oauth/token request via HTTP like the example shows, that it returns the "Checking your browser" page HTML markup every time. However, if I try the same request over HTTPS, it returns the expected payload with the access token.

Note I am not an engineer and not well versed in Oauth, this is just something that absolutely didn't work until I tried HTTPS and then worked as expected. It might be a good idea to have an engineer with an understanding of Oauth verify this.

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group label, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
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  • Add docs-only when the only files changed are under doc/*.
  • Add twdoing when starting work on the MR.
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3. Maintainer

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