add `()` for inline functions and fix typos

[zekiakyol]

Thank you for this great book!

This pull request

• Adds () for all inline functions, cover-to-cover
• Changes one inline group_by() to groupby() at https://aeturrell.github.io/coding-for-economists/data-numbers.html?highlight=group_by#counts
• Adds a missing math mode at https://aeturrell.github.io/coding-for-economists/econmt-regression.html#high-dimensional-fixed-effects-aka-absorbing-regression by changing \gamma to $\gamma$
• Fixes multiple incorrectly centered \hat usage in math mode at https://aeturrell.github.io/coding-for-economists/econmt-regression.html by updating: $\hat{x_{2i}}$ to $\hat{x}_{2i}$
• Changes one inline touch to touches() at https://aeturrell.github.io/coding-for-economists/geo-intro.html#neighbours
• Fixes puncution.

[zekiakyol]

Thank you!