Fix wrong syntax in doc for Ingress Secret Opaque for required argument "fields"

[SRodi]

Fix required argument "fields" in Kubernetes Service container_ingress_secret_opaque resource documentation.

This is to prevent the following error:

❯ terraform plan
╷
│ Error: Unsupported argument
│
│   on main.tf line 18, in resource "ibm_container_ingress_secret_opaque" "secret":
│   18:   fields = {
│
│ An argument named "fields" is not expected here. Did you mean to define a block of type
│ "fields"?
╵

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Relates OR Closes #0000

Output from acceptance testing:

n/a