Fix fontification of outdated TeX form

Fix fontification of outdated TeX form

* lisp/textmodes/tex-mode.el (tex-font-lock-keywords-2): End the
expression before the terminating $ in constructions like $\it
identifiername$
(bug#28277). This avoids italicising the final $ character.

This fixes the final $ of the final test case here:

$foo$
\textit{foo}
{\it foo}
$\mathit{identifiername}$
$\textit{identifiername}$
${\it identifiername}$
$\it identifiername$