Fix fontification of outdated TeX form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}
$\mathit{identifiername}$
$\textit{identifiername}$
${\it identifiername}$
$\it identifiername$

lisp/textmodes/tex-mode.el

@@ -593,7 +593,7 @@ An alternative value is \" . \", if you use a font with a narrow period."
 	    ;; Miscellany.
 	    (slash "\\\\")
 	    (opt " *\\(\\[[^]]*\\] *\\)*")
-	    (args "\\(\\(?:[^{}&\\]+\\|\\\\.\\|{[^}]*}\\)+\\)")
+	    (args "\\(\\(?:[^${}&\\]+\\|\\\\.\\|{[^}]*}\\)+\\)")
 	    (arg "{\\(\\(?:[^{}\\]+\\|\\\\.\\|{[^}]*}\\)+\\)"))
        (list
 	;;