Update sites/handbook/source/handbook/engineering/ux/technical-writing/fundamentals/index.html.md to clarify who can access edcast technical writing course

To suggest a change to the Product Handbook, please review the updating the Product Handbook section.

Related to: #13376 (closed)

Please provide a brief explanation for this change:

The course linked from https://about.gitlab.com/handbook/engineering/ux/technical-writing/fundamentals/ is only available to GitLab team members. This MR tries to make this clear so that contributors who are not GitLab team members don't spend time trying to login to a system they cannot access.

Please indicate the types of revisions being suggested for the Product Handbook (please check all that apply):

• Small improvement (typos, clarifications, etc.)
• Adding a new section
• Modifying existing section
• Documenting a new process
• Adding a new page or directory
• Other

Please indicate Milestone

• Assigned to Milestone

Author Checklist

• Provided a concise title for the MR
• Provided a brief explanation for this change (Say why, not just what)
  • (Attach screenshots, Slack conversations, etc. as necessary)
• Indicated the types of changes included in this MR
• Verified that no confidential data is in this MR
• Assigned reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #product or #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.