[lec17] fix typos in the analysis of gradient descent

* the lower bound using the Hessian (eq. 9, 10) had a minus instead of a plus
* reducing the error by a factor \eps requires O(\log 1/\eps) iterations (and
not O(\log \eps)
Merge pull request #9 from Thibauth/master

[lec17] fix typos in the analysis of gradient descent