v2: Migration.md extraConfig erroneous v2 path

[marcomusso]

When migrating from v1 to v2 the Migration guide states that the extraConfig keys is now scoped under the alloy-XXXX keys but the table at the beginning of the paragraph seems a copy/paste of the integration one, ie the previous paragraph.

In short: "Steps to take" is correct but the table is not.

Reference: https://github.com/grafana/k8s-monitoring-helm/blob/main/charts/k8s-monitoring/docs/Migration.md#extra-configs