Fix typos and update plots

[carschno]

There are some (minor) issues in episode 3

Section 6 (Train the model)

• the plot does not show the y label as defined in the plot_history() function.
• update the plot to show the actual data
• (optional) plot both RMSE and loss for comparison

Section 8 (Measure the performance)

• Update evaluation output numbers

Section 9 (Refine the model)

• Update plot for added Batchnorm layer

Keypoints

• there are more key points than the ones currrently listed