docs: fix README app.kubernetes.io/instance issue

[blue86321]

Description:

In README ### Configure resource attributes with labels section,
claims that label app.kubernetes.io/instance will become service.instance, which is incorrect.

Label app.kubernetes.io/instance will indeed become service.name as in #3797

This logic is still valid if we go deep into the code

// pkg/constants/env.go
LabelAppName = []string{
  "app.kubernetes.io/instance",
  "app.kubernetes.io/name",
}

// internal/instrumentation/sdk.go # chooseServiceName
if name := chooseLabelOrAnnotation(pod, useLabelsForResourceAttributes, semconv.ServiceNameKey, constants.LabelAppName); name != "" {
  return name
}

#4081 misunderstood and falsely "fixed the typo". This PR is to re-fix this issue in README.md

[blue86321]

@frzifus this PR somehow get stuck by unit test. I just rebase the latest code and push again.

stucked CI check: https://github.com/open-telemetry/opentelemetry-operator/actions/runs/17571522619