Release the webcam VideoCapture in demo.py
Release the webcam VideoCapture in demo.py

Summary:
As per the documentation of OpenCV, one should free the allocated resources (using VideoCapture.release()) when done with the webcam.

The current demo.py forgets to do this. This pull request fixes it.
Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/1411

Reviewed By: rbgirshick

Differential Revision: D21540644

Pulled By: ppwwyyxx

fbshipit-source-id: 8df344007a7349084f6f256d80ca74ace23a28b6

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2