The varible `TeX-mode-local-vars-hook` is not called correctly as AucTeX
reports that the major mode in a LaTeX file is `latex-mode`. Instead,
let's use `latex-mode-local-vars-hook` to enable flyspell.
This change enables disabling the feature of flyspell of immediately
spellchecking a document with:
`(setq-hook! 'TeX-mode-hook +spellcheck-immediately nil)`