Fix broken link in getting-started documentation

[r8ygun]

Link to Promtail didn't work.

What this PR does / why we need it:

Makes navigating the getting started guide easier.

Special notes for your reviewer:

Checklist

• [x ] Documentation added
• Tests updated
• Add an entry in the CHANGELOG.md about the changes.

[CLAassistant]

All committers have signed the CLA.

[stale]

Hi! This issue has been automatically marked as stale because it has not had any
activity in the past 30 days.

We use a stalebot among other tools to help manage the state of issues in this project.
A stalebot can be very useful in closing issues in a number of cases; the most common
is closing issues or PRs where the original reporter has not responded.

Stalebots are also emotionless and cruel and can close issues which are still very relevant.

If this issue is important to you, please add a comment to keep it open. More importantly, please add a thumbs-up to the original issue entry.

We regularly sort for closed issues which have a stale label sorted by thumbs up.

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• Mark issues as revivable if we think it's a valid issue but isn't something we are likely
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[r8ygun]

This change is likely going to register the the lowest possible in terms of things you want to actually spend time on, so no worries if it's closed/marked as stale.

[KMiller-Grafana]

@r8ygun, I'm actually revising/rewriting the Loki getting started guide right now. I'll make sure that the links are correct as part of the effort. Right now, I'm working in draft form, but you can see some of my initial commits in #5939. I think that you can close this PR. Thanks.

[stale]

Hi! This issue has been automatically marked as stale because it has not had any
activity in the past 30 days.

We use a stalebot among other tools to help manage the state of issues in this project.
A stalebot can be very useful in closing issues in a number of cases; the most common
is closing issues or PRs where the original reporter has not responded.

Stalebots are also emotionless and cruel and can close issues which are still very relevant.

If this issue is important to you, please add a comment to keep it open. More importantly, please add a thumbs-up to the original issue entry.

We regularly sort for closed issues which have a stale label sorted by thumbs up.

We may also:

• Mark issues as revivable if we think it's a valid issue but isn't something we are likely
  to prioritize in the future (the issue will still remain closed).
• Add a keepalive label to silence the stalebot if the issue is very common/popular/important.

We are doing our best to respond, organize, and prioritize all issues but it can be a challenging task,
our sincere apologies if you find yourself at the mercy of the stalebot.