fix doc "collect/metamonitoring": relocate 'replace prometheus.exporter.self "<SELF_LABEL>" instruction' to correct location

[edv1n]

PR Description

fix doc collect/metamonitoring that:

The instruction of

Replace the following:

• <SELF_LABEL>: The label for the component such as default or metamonitoring. The label must be unique across all prometheus.exporter.self components in the same configuration file.

should be under the code block of

   prometheus.exporter.self "<SELF_LABEL>" {
   }

instead of the code block of

   prometheus.scrape "<SCRAPE_LABEL>" {
     targets    = prometheus.exporter.self.<SELF_LABEL>.targets
     forward_to = [<METRICS_RECEIVER_LIST>]
   }

Which issue(s) this PR fixes

This PR is a trivial fix, no issue is required according to the Contributing guideline

Notes to the Reviewer

I found the instruction of replacing the <SELF_LABEL> under step 2 is a bit difficult to follow, so I suggest changing it to step 1. I have also checked the doc of collect/datadog-traces-metrics to ensure consistency.

PR Checklist

I am not sure if I need to update the CHANGELOG.md as this is just a small fix of the documentation.

[clayton-cornell]

Thanks for catching this. Approving and merging.