text: Title: Typo in 4.1
  Body: https://github.com/Yorko/mlcourse.ai/blob/master/jupyter_english/topic04_linear_models/topic4_linear_models_part1_mse_likelihood_bias_variance.ipynb
  k should be renamed to x.
  ![image](https://user-images.githubusercontent.com/22998537/45756573-dfd8ab80-bc29-11e8-9412-249940eb0a35.png)
  P.S. Maybe you should change the '|' notation. It got me confused because I thoutght that it is conditional probability and had to search for additional information
  P.S.S Also for some reason the maximum likelihood method is not finished and resulting theta is not found after finding the derivative