[expr.post] Fix scope of \opt markup (#4142)

Author: languagelawyer

source/expressions.tex

@@ -2823,8 +2823,8 @@
     typename-specifier \terminal{(} \opt{expression-list} \terminal{)}\br
     simple-type-specifier braced-init-list\br
     typename-specifier braced-init-list\br
-    postfix-expression \opt{\terminal{.} \terminal{template}} id-expression\br
-    postfix-expression \opt{\terminal{->} \terminal{template}} id-expression\br
+    postfix-expression \terminal{.} \opt{\terminal{template}} id-expression\br
+    postfix-expression \terminal{->} \opt{\terminal{template}} id-expression\br
     postfix-expression \terminal{++}\br
     postfix-expression \terminal{-{-}}\br
     \keyword{dynamic_cast} \terminal{<} type-id \terminal{>} \terminal{(} expression \terminal{)}\br