[UPDATE]Typo in Uint3 "Train our Deep Q-Learning Agent to Play Space Invaders " Section

The explanation of the given example `dqn.yml` file in **Unit3** _Train our Deep Q-Learning Agent to Play Space Invaders_ Section has a typo in the 3rd item.

In line 6 of `dqn.yml`, it writes `n_timesteps: !!float 1e6`. But in the third item of the explanation it writes, "We train it for 10 million `n_timesteps`" where 10 million equals 1e7.