Fix dpctl.tensor.usm_ndarray.flags docstring

This makes it link to the new flags documentation

Author: ndgrigorian

dpctl/tensor/_usmarray.pyx

@@ -661,7 +661,7 @@ cdef class usm_ndarray:
     @property
     def flags(self):
         """
-        Returns :class:`dpctl.tensor._flags` object.
+        Returns :class:`dpctl.tensor._flags.Flags` object.
         """
         return _flags.Flags(self, self.flags_)