Fixed rocketchat_configs docs

[rekup]

This MR fixes a typo in the documentation regarding the new rocketchat receiver. The field is called rocketchat_configs instead of rocket_configs: https://github.com/prometheus/alertmanager/blob/main/config/config.go#L1028.

As per the current documentation the configuration should look like this:

# ...
receivers:
- name: test
  rocket_configs:
    - api_url: https://chat-stg.example.org
      channel: prom-test
      token: redacted
      token_id: redacted

However, this results in the following error:

Jan 23 09:33:59 prometheus01.example.org alertmanager[521194]: time=2025-01-23T08:33:59.255Z level=INFO source=main.go:191 msg="Starting Alertmanager" version="(version=0.28.0, branch=HEAD, revision=4ce04fb010bd626fca35928dcfe82f6f2da52ced)"
Jan 23 09:33:59 prometheus01.example.org alertmanager[521194]: time=2025-01-23T08:33:59.255Z level=INFO source=main.go:192 msg="Build context" build_context="(go=go1.23.4, platform=linux/amd64, user=root@40be7f318ba7, date=20250115-14:22:34, tags=netgo)"
Jan 23 09:33:59 prometheus01.example.org alertmanager[521194]: time=2025-01-23T08:33:59.289Z level=ERROR source=coordinator.go:117 msg="Loading configuration file failed" component=configuration file=/etc/alertmanager/alertmanager.yml err="yaml: unmarshal errors:\n  line 27: field rocket_configs not found in type config.plain"

The correct config should look like this:

# ...
receivers:
- name: test
  rocketchat_configs:
    - api_url: https://chat-stg.example.org
      channel: prom-test
      token: redacted
      token_id: redacted