Cost function does not render theta correctly in ex1 ipynb

[amandazhuyilan]

The algorimic expression for Cost Function is rendering theta as an "Unknown Character":

My guess is that you'll just have to replace the "Θ" character to \theta in the algorimic expression to have ipynb render the greek character properly.

P.S - Great work for the CS229 assignment and solutions - well organized and clearly described! Your repo is certainly one of the best I have seen around!