Fix 'grafana-agent' URL on Kubernetes docs

[myhro]

As reported on #466, the documented Kubernetes DaemonSet manifest is lacking the http:// prefix for the grafana-agent service URL, which results in the following error:

time=2023-12-04T19:17:20.297Z level=ERROR msg="Beyla couldn't start read and
forwarding" error="can't instantiate instrumentation pipeline: instantiating
terminal instance \"Metrics\": instantiating OTEL metrics reporter: can't
instantiate OTEL HTTP metrics exporter: URL \"grafana-agent:4318\" must have a
scheme and a host"

Closes #466.

[CLAassistant]

All committers have signed the CLA.

[mariomac]

Amazing! thank you for your contribution