[incubator/kafka] fix commands documentation in NOTES.txt

[zjgoodman]

Is this a new chart

No

What this PR does / why we need it:

The existing Kafka chart's NOTES.txt has incorrect instructions.

Here's a short snippet of the post-install rendered notes

$ helm install my-kafka --namespace kafka incubator/kafka
NAME: my-kafka
LAST DEPLOYED: Tue Apr 28 12:18:59 2020
NAMESPACE: kafka
STATUS: deployed
REVISION: 1
NOTES:
### Connecting to Kafka from inside Kubernetes
...

Once you have the testclient pod above running, you can list all kafka
topics with:

  kubectl -n kafka exec testclient -- kafka-topics --zookeeper my-kafka-zookeeper:2181 --list

...

I copy/pasted the command provided to list kafka topics and this was the output

$ kubectl -n kafka exec testclient -- kafka-topics --zookeeper my-kafka-zookeeper:2181 --list
OCI runtime exec failed: exec failed: container_linux.go:346: starting container process caused "exec: \"kafka-topics\": executable file not found in $PATH": unknown
command terminated with exit code 126

Because the Kafka docker image's workdir is /opt/kafka and the binaries are in /opt/kafka/bin, the relative path './bin/' needs to be prepended to the binary calls in the kubectl exec commands

Which issue this PR fixes

N/A

Special notes for your reviewer:

N/A

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
  • Should the version be incremented for a doc change like this?
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [stable/mychartname])

[k8s-ci-robot]

Hi @zjgoodman. Thanks for your PR.

I'm waiting for a helm member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[zjgoodman]

/assign @benjigoldberg

[zjgoodman]

@unguiculus @benjigoldberg hey there, just wondering if this PR needs anything else from me before it can be reviewed & merged. This is my first time contributing here so I just wanna make sure everything's in order 👌

[davidkarlsen]

/ok-to-test
/assign
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: davidkarlsen, zjgoodman

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• incubator/kafka/OWNERS [davidkarlsen]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment