py: fix error msg for type 3 plan

python/cufinufft/cufinufft/_plan.py

@@ -117,7 +117,7 @@ def __init__(self, nufft_type, n_modes, n_trans=1, eps=1e-6, isign=None,
                 dim = n_modes
                 n_modes = (1,) * dim     # Ignored, can be anything
             else:
-                raise ValueError("Type 3 plan must have n_modes_or_dim equal to the dimension")
+                raise ValueError("For a type 3 plan, n_modes_or_dim must be a single number, the dimension")
         else:
             if isinstance(n_modes, numbers.Integral):
                 n_modes = (n_modes,)