Description

This PR fixes the broken link in the Prometheus exporter README.rst by updating it to point to the current documentation location on opentelemetry.io.

The old link pointed to a non-existent page on readthedocs:
https://opentelemetry-python.readthedocs.io/en/latest/exporter/prometheus/prometheus.html

The new link points to the current documentation:
https://opentelemetry.io/docs/instrumentation/python/exporters/#prometheus

Fixes #4399

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Verified that the new link is accessible and points to the correct documentation
• Checked that the RST formatting is correct in the README file

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated