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

@pdpinch @carsongee  This is the same correction to URL that Ned requested in pylti.
Yes, it would render as a single block, but that is what makes sense for copy paste commands in general.  You could actually replace this whole block with a single bash block since you are basically documenting a small shell script already.  None of this is required for merge, just my unasked for opinion ;)