Clarify PlacementRef namespace field requirement

Fix confusing explanation on line 638. The namespace field is required
because multiple Placements with the same name can exist in different
namespaces, not simply because SubjectMapping is cluster-scoped.

The new explanation directly states the reason for requiring the
namespace field.

Signed-off-by: zhujian <jiazhu@redhat.com>

Author: zhujian7

enhancements/sig-architecture/233-subject-mapping/README.md

@@ -635,7 +635,7 @@ type PlacementRef struct {
     Name string `json:"name"`
 
     // Namespace is the namespace of the Placement resource
-    // Required since SubjectMapping is cluster-scoped
+    // Required since multiple Placements with the same name can exist in different namespaces
     // +required
     // +kubebuilder:validation:MinLength=1
     Namespace string `json:"namespace"`