[class.access.base] Add missing formatting to access specifier

Author: geryogam

source/classes.tex

@@ -4748,7 +4748,7 @@
 public and protected members of the base class are accessible as
 protected members of the derived class.
 If a class is declared to be a base class for another class using the
-private
+\tcode{private}
 access specifier, the
 public and protected
 members of the base class are accessible as private