Fix pip install extras command in CONTRIUTING doc (#4515)
Fix pip install extras command in CONTRIUTING doc (#4515)

Before this change, the document listed `pip install -e . [extras]`. This command did not work. When I removed the space between `.` and `[extras]`, like the same command below it, the command worked.

Signed-off-by: Jerry Liang <e2e2a8898401ec353231880ea2ea962b40218aa8@databricks.com>
mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow