Fix broken link to storage driver testing docs
Problem
The link to docs-gitlab/storage-driver-integration-testing-guide.md
in docs-gitlab/README.md
is broken.
This can be tested in https://gitlab.com/gitlab-org/container-registry/tree/3118cd438a2fb4477aba06c64871f2fd2c35c03b/docs-gitlab, by clicking in Local Integration Testing.
Solution
Remove the path from the URL so that GitLab can resolve it.