docs: Broken link in README.md

`environment_url.txt` file.

Therefore if an upstream job in the pipeline deployed your code to a server and propagated the deployed server url,

either through a [dotenv](https://docs.gitlab.com/ee/ci/pipelines/job_artifacts.html#artifactsreportsdotenv) variable `$environment_url` 

either through a [dotenv](https://docs.gitlab.com/ee/ci/yaml/artifacts_reports.html#artifactsreportsdotenv) variable `$environment_url` 

or through a basic `environment_url.txt` file, then the User centric Usecases Validator test will automatically be run on this server.

:warning: all our deployment templates implement this design. Therefore even purely dynamic environments (such as review