Changed link to github.com/mitodl/mit-lti-flask-sample

@pdpinch @carsongee  This is the same correction to URL that Ned requested in pylti.
Optional:

I prefer copy-and-pastable commands, but I'm not sure it's worth the effort for something so basic. I suppose you could change it to:

1. Clone this repository and navigate to its directory:
``git clone git@github.com:mitodl/mit_lti_flask_sample.git``
``cd mit_lti_flask_sample``

(not sure I have the rst exactly right)