Merge pull request #2077 from camptocamp/backport/2076-to-6.0

[Backport 6.0] Fix doc - prometheus port var env name

README.md

@@ -420,7 +420,7 @@ command line. Usually done in the [Dockerfile](acceptance_tests/app/Dockerfile)
 It will work in multi process mode with the limitation listed in the
 [`prometheus_client` documentation](https://github.com/prometheus/client_python#multiprocess-mode-eg-gunicorn).
 
-To enable it you should provide the `C2CWSGIUTILS_PROMETHEUS_PORT` environment variable.
+To enable it you should provide the `C2C_PROMETHEUS_PORT` environment variable.
 For security reason, this port should not be exposed.
 
 We can customize it with the following environment variables: