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

@pdpinch @carsongee  This is the same correction to URL that Ned requested in pylti.
why not just make this one block, i.e.
```
.. code-block:: bash

git clone git@github.com:mitodl/mit_lti_flask_sample.git
cd mit_lti_flask_sample

```