implemented __str__ for BomRef

Signed-off-by: Paul Horton <phorton@sonatype.com>

Author: madpah

cyclonedx/model/bom_ref.py

@@ -50,4 +50,7 @@ def __hash__(self) -> int:
         return hash(self.value)
 
     def __repr__(self) -> str:
+        return f'<BomRef {self.value}'
+
+    def __str__(self) -> str:
         return self.value