Typo fix

Doc/lib/libstdtypes.tex

@@ -7,7 +7,7 @@ differed from user-defined types because it was not possible to use
 the built-in types as the basis for object-oriented inheritance.
 This limitation does not exist any longer.}
 
-The principal built-in types are numerics, sequences, mappings, files
+The principal built-in types are numerics, sequences, mappings, files,
 classes, instances and exceptions.
 \indexii{built-in}{types}