Updated the variable explicitly for allowed hosts to mlflow server in demo-3-mlflow

[raywangau]

• Fixed the issue 'Invalid Host header - possible DNS rebinding attack detected' preventing localhost:5001 (and 127.0.0.1:5001) from being loaded properly for mlflow_server in demo-3-mlflow.
• Fixed the README.md filename with trailing space removed (which prevented the Markdown preview) in demo-3-mlflow.