Fix typo in unet_blocks.py (huggingface#353)

Update unet_blocks.py

fix typo

src/diffusers/models/unet_blocks.py

@@ -60,7 +60,7 @@ def get_down_block(
         )
     elif down_block_type == "CrossAttnDownBlock2D":
         if cross_attention_dim is None:
-            raise ValueError("cross_attention_dim must be specified for CrossAttnUpBlock2D")
+            raise ValueError("cross_attention_dim must be specified for CrossAttnDownBlock2D")
         return CrossAttnDownBlock2D(
             num_layers=num_layers,
             in_channels=in_channels,