docs: fix typo in example code (#588)

"nbd" -> "ndb"

Thanks

Author: dp-rufus

packages/google-cloud-ndb/google/cloud/ndb/model.py

@@ -941,7 +941,7 @@ class WidgetProperty(ndb.Property):
             def _validate(self, value):
                 # Lax user value to strict user value.
                 if not isinstance(value, Widget):
-                    raise nbd.exceptions.BadValueError(value)
+                    raise ndb.exceptions.BadValueError(value)
 
             def _to_base_type(self, value):
                 # (Strict) user value to base value.