Update virtualenv generation

https://piazza.com/class/j0vi72697xc49k?cid=1760

Author: omoindrot

assignments/2017/assignment3.md

@@ -63,7 +63,7 @@ If you decide to work locally, we recommend using [virtual environment](http://d
 ```bash
 cd assignment2
 sudo pip install virtualenv      # This may already be installed
-python3 -m venv .env       		 # Create a virtual environment (python3)
+virtualenv -p python3 .env   		 # Create a virtual environment (python3)
 source .env/bin/activate         # Activate the virtual environment
 pip install -r requirements.txt  # Install dependencies
 # Note that this does NOT install TensorFlow or PyTorch,