Python Virtual Environment command in README.md is incorrect

[shunphoenix55]

Start a python virtual env:

# navigate to the flask-pymongo-example directory
cd flask-pymongo-example
# create the virtual environment for MFlix
python3 -m venv mflix-venv
# activate the virtual environment
source mflix_venv/bin/activate

In this code, line 4 has mflix-venv while line 6 has mflix_venv (first is a hyphen, second is an underscore)