Update typesystem.pod6 in Class section for minor grammar fix

Author: adaptiveoptics

doc/Language/typesystem.pod6

@@ -75,8 +75,8 @@ compiler will check at the end of the current scope if the type is defined.
 
 A I<class> is a stereotype that expresses a specific type that can be instantiated
 into I<objects> (also called I<instances>). Each object (instance of the class)
-has access to the same common I<behaviour> defined in the class itself, while
-can keep an internal state not shared with other instances of the same class.
+has access to the same common I<behaviour> defined in the class itself, while keeping
+an internal state not shared with other instances of the same class.
 The behavioural part of each object is defined in the class using I<methods>,
 while the internal state is provided via I<attributes> (variable scoped within the
 class).