[temp.res.general] Fix nesting for \terminal{\opt{...}} (#7599)

Author: ilazaric

source/templates.tex

@@ -4515,7 +4515,7 @@
 \begin{bnf}
 \nontermdef{typename-specifier}\br
   \keyword{typename} nested-name-specifier identifier\br
-  \keyword{typename} nested-name-specifier \terminal{\opt{template}} simple-template-id
+  \keyword{typename} nested-name-specifier \opt{\terminal{template}} simple-template-id
 \end{bnf}
 
 \pnum