Update tanka.md so that promtail.yml is the correct format

[shokada]

Following https://github.com/grafana/loki/blob/master/docs/installation/tanka.md, I encountered the following error.

% kubectl logs promtail-pf4z7
Unable to parse config: /etc/promtail/promtail.yml: yaml: unmarshal errors:
  line 2: field container_root_path not found in type client.raw
  line 3: field hostname not found in type client.raw
  line 4: field password not found in type client.raw
  line 5: field scheme not found in type client.raw
  line 7: field username not found in type client.raw

The content of promtail.yml is the following.

% tk show environments/loki --dangerous-allow-redirect | grep -A8 'promtail.yml: |'
  promtail.yml: |
    clients:
    - container_root_path: /var/lib/docker
      hostname: gateway.loki.svc
      password: password
      scheme: http
      url: http://loki:password@gateway.loki.svc/loki/api/v1/push
      username: loki
    scrape_configs:

So, I use :: so that each field doesn't appear in the JSON output.

[claassistantio]

All committers have signed the CLA.

[rfratto]

LGTM, thanks!