docs(Dockerfile): Fix example of usage on Dockerfile (#545)

* Closes #544

Co-authored-by: mercuriete <mercuriete@gmail.com>

Dockerfile

@@ -12,6 +12,6 @@ ENTRYPOINT ["gitleaks"]
 # How to use me :
 
 # docker build -t gitleaks .
-# docker run --rm --name=gitleaks gitleaks --repo=https://github.com/zricethezav/gitleaks
+# docker run --rm --name=gitleaks gitleaks --repo-url=https://github.com/zricethezav/gitleaks
 
 # This will check for secrets in https://github.com/zricethezav/gitleaks