Fixed typos in reconst/dti.py

Stumbled in a small latex typo in the doc, I had forgotten to add a \ in front of eigen_vectors in the formula. I also rewrote the color_fa as a one liner and fixed another small typo in the doc of the function underneath.

Shouldn't break anything either, but I didn't run the code (except for a really basic ipython test, to see if the syntax was still good) . It's only a small rewrite, so there shouldn't be any mistake.