[class.virtual] Add \keyword markup for 'final' and 'override' (#4139)

This indexes the two paragraphs that actually specify the rules for
'final' and 'override' applied to functions.

Author: AlisdairM

source/classes.tex

@@ -3850,7 +3850,7 @@
 
 \pnum
 If a virtual function \tcode{f} in some class \tcode{B} is marked with the
-\grammarterm{virt-specifier} \tcode{final} and in a class \tcode{D} derived from \tcode{B}
+\grammarterm{virt-specifier} \keyword{final} and in a class \tcode{D} derived from \tcode{B}
 a function \tcode{D::f} overrides \tcode{B::f}, the program is ill-formed.
 \begin{example}
 \begin{codeblock}
@@ -3865,7 +3865,7 @@
 \end{example}
 
 \pnum
-If a virtual function is marked with the \grammarterm{virt-specifier} \tcode{override} and
+If a virtual function is marked with the \grammarterm{virt-specifier} \keyword{override} and
 does not override a member function of a base class, the program is ill-formed.
 \begin{example}
 \begin{codeblock}