Typo in 2.3.1

[FenglongSong]

Hi Harry,

Thanks for taking these notes! They are clear and very helpful. I found a typo in 2.3.1, which should be $\pi(a_t | o_1, ..., o_t)$ rather than $\pi(a_t | o+1, ..., o_t)$

It should be a easy fix in L20 in https://github.com/harryzhangOG/Deep-RL-Notes/blob/master/imitation.tex

Thanks a lot!

[Jah-yee]

@harryzhangOG I've created PR #7 fixing this typo: o+1 → o_1 in imitation.tex line 20. The fix corrects the notation to properly represent the sequence of observations from time step 1 to t.