Some small typos

[juanitorduz]

Here is a small PR with some small typos and improvements.

• 18-Heterogeneous-Treatment-Effects-and-Personalization

Fixed LaTex delta -> \delta

• 22-Debiased-Orthogonal-Machine-Learning:

From

Then, for each part k, we will estimate the ML models on all the other K-k samples and make the residuals on the k part.

To

Then, for each part k, we will estimate the ML models on all the other K-1 samples and make the residuals on the k part.

• 24-The-Diff-in-Diff-Saga.

Typo rolot to rollout

[juanitorduz]

• 25-Synthetic-Diff-in-Diff. Typo Cynthetic -> Synthetic (see commit https://github.com/matheusfacure/python-causality-handbook/pull/271/commits/eb2e79423ecfa012e7dc2630aea9eb600cd8d217)

[matheusfacure]

Thanks a lot, Juan!