TST: fix incorrect dtype in test

Author: ngoldbaum

numpy/_core/tests/test_stringdtype.py

@@ -766,7 +766,7 @@ def test_multiply_reduce():
 
 
 def test_multiply_two_string_raises():
-    arr = np.array(["hello", "world"])
+    arr = np.array(["hello", "world"], dtype="T")
     with pytest.raises(np._core._exceptions._UFuncNoLoopError):
         np.multiply(arr, arr)