Update kubernetes.md with correct type for azure cache.

What does this MR do?

Fix copy-paste-error in docs. Type should be "azure", not "s3" for the azure cache.

Why was this MR needed?

I copy pasted the example code for Azure cache, and it didn't work.

I was getting errors like this in my runners logs:

ERROR: error while generating S3 pre-signed URL error=Bucket name cannot be empty

On this page it is shown with value "azure": https://gitlab.com/gitlab-org/gitlab-runner/-/blob/master/docs/configuration/advanced-configuration.md#the-runnerscacheazure-section

What's the best way to test this MR?

Read.

What are the relevant issue numbers?

N/A