Add explicit main to pull loki and promtail to install it via Tanka (…

…#3848)

Rationale is, by default `jb` command, assumes `master` branch, but Loki repo renamed
the `master` to `main` branch.

docs/sources/installation/tanka.md

@@ -31,8 +31,8 @@ Download and install the Loki and Promtail module using `jb`:
 ```bash
 go get -u github.com/jsonnet-bundler/jsonnet-bundler/cmd/jb
 jb init  # not required if you already ran `tk init`
-jb install github.com/grafana/loki/production/ksonnet/loki
-jb install github.com/grafana/loki/production/ksonnet/promtail
+jb install github.com/grafana/loki/production/ksonnet/loki@main
+jb install github.com/grafana/loki/production/ksonnet/promtail@main
 ```
 
 Then you'll need to install a kubernetes library: