Clarify whose are constraints.

According to @moritz's comment in the IRC conversation pointed from
the issue. Closes #1217.

Author: JJ

doc/Language/containers.pod6

@@ -324,7 +324,7 @@ last resort, you can use Promises to L<handle|/type/Promise#method_in> timeouts.
 Any container can have a type constraint in the form of
 a L<type object|/language/typesystem#Type_objects> or a L<subset|/language/typesystem#subset>.
 Both can be placed between a declarator and the variable name or after the trait L<of|/type/Variable#trait_is_dynamic>.
-The constraint is a property of the container, not the variable.
+The constraint is a property of the variable, not the container.
 
 Variables may have no container in them, yet still offer the ability to
 re-bind and typecheck that rebind. The reason for that is in such cases the