Docs: fix link to chart/flux/README.md

[ttarczynski]

I've found that links to chart/flux/README.md are broken when accessing from https://docs.fluxcd.io

The 3 broken links are:

1. https://docs.fluxcd.io/en/latest/tutorials/get-started-helm.html
   links to https://fluxcd/ and should link to https://github.com/fluxcd
2. https://docs.fluxcd.io/en/latest/tutorials/get-started-helm.html
   renders without the .md
<a class="reference external" href="https://github.com/fluxcd/flux/blob/master/chart/flux/README">in the chart’s README</a>.</p>
3. https://docs.fluxcd.io/en/latest/troubleshooting.html
   renders without the .md
<a class="reference external" href="https://github.com/fluxcd/flux/blob/master/chart/flux/README">helm chart</a>

[hiddeco]

Thank you @ttarczynski 🌷