Small change in docstring about domain parameter

src/operators/matrix_product_operator.py

@@ -47,7 +47,7 @@ class MatrixProductOperator(EndomorphicOperator):
     ----------
     domain: :class:`Domain` or :class:`DomainTuple`
         Domain of the operator.
-        If :class:`DomainTuple` it is assumed to have only one entry.
+        If :class:`Domain` it is assumed to have only one subspace.
     matrix: scipy.sparse matrix or numpy array
         Quadratic matrix of shape `(domain.shape, domain.shape)`
         (if `not flatten`) that supports `matrix.transpose()`.