Docs: update Loki Helm Chart document to support Helm 3

[oke-py]

What this PR does / why we need it:

This PR updates Loki Helm Chart document to support Helm 3.
-n flag has been deprecated for helm install.

Which issue(s) this PR fixes:
Fixes #1690

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated

[cyriltovena]

Hello and thank you !

Does that still work with 2.0 helm ? I don't think it does so many we should add instruction for helm 3 instead or replacing with 2.0 what do you think ?

[oke-py]

The command that I updated does not work with Helm 2.

$ helm2 install loki-grafana stable/grafana -n loki 
Error: This command needs 1 argument: chart name

To install the chart in a specified namespace, the command is as below.

$ helm2 install stable/grafana -n loki-grafana --namespace loki

Given that Helm 3.0 was released about a half year ago, I think it'll be enough to support Helm 3.
However, it might be sure many developers are still using Helm 2.
How about showing both commands?

# with Helm 2
$ helm install stable/grafana -n loki-grafana --namespace <YOUR-NAMESPACE>

# with Helm 3
$ helm install loki-grafana stable/grafana -n <YOUR-NAMESPACE>

[cyriltovena]

Yes let’s do that !

[oke-py]

updated!

[codecov-io]

Codecov Report

Merging #2038 into master will decrease coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2038      +/-   ##
==========================================
- Coverage   64.05%   63.88%   -0.17%     
==========================================
  Files         133      133              
  Lines       10217    10217              
==========================================
- Hits         6544     6527      -17     
- Misses       3185     3203      +18     
+ Partials      488      487       -1     

Impacted Files	Coverage Δ
pkg/promtail/positions/positions.go	47.32% <0.00%> (-13.40%)	⬇️
pkg/promtail/targets/tailer.go	76.13% <0.00%> (-2.28%)	⬇️

[cyriltovena]

Thank for taking care of the feedback ! LGTM