use different python version in virtualenv code example

Example 1: uses specific version python venv

# Use python version 3.7.3 for tensorflow
pip install virtualenv 
virtualenv -p PYTHONPATH <path/to/new/virtualenv/>

Example 2: virtualenv specify python version

virtualenv --python=/usr/bin/python2.6 <path/to/new/virtualenv/>

Example 3: start virtualenv with python version

python3 -m venv <myenvname>