Fix pip install extras command in CONTRIUTING doc (mlflow#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 <jerry.liang@databricks.com>

CONTRIBUTING.rst

@@ -140,7 +140,7 @@ by running the following from your checkout of MLflow:
 
     conda create --name mlflow-dev-env python=3.6
     conda activate mlflow-dev-env
-    pip install -e . [extras] # installs mlflow from current checkout with some useful extra utilities
+    pip install -e .[extras] # installs mlflow from current checkout with some useful extra utilities
 
 If you plan on doing development and testing, you will also need to install the following into the conda environment: