Error:
```
$> kubectl exec -ti redis-with-volume-secrets /bin/bash
kubectl exec [POD] [COMMAND] is DEPRECATED and will be removed in a future version. Use kubectl exec [POD] -- [COMMAND] instead.
error: unable to upgrade connection: container not found ("redis")
```
* fix: fix internal service filename in 08-service section
* fix: typo
* fix: fix markdownlint in CI
* fix: typo
* fix: typo
* fix: `environment variables`, not `environmental`
* fix: typo & missing punctuation
* fix: `much` not `many`
* fix: `lets` not `let's`
* fix: typo
* fix: typo
* fix: phrasing
* fix: typo
* fix: typo
* fix: mysql operator manifest api version
got this error while trying to run it as is:
error: unable to recognize "20-operators/01-mysql-operator.yml": no matches for
kind "Deployment" in version "apps/v1beta1"
* fix: spelling