fix typo docstring in unet2d (#798)

fix typo docstring

src/diffusers/models/unet_2d_condition.py

@@ -223,7 +223,7 @@ class UNet2DConditionModel(ModelMixin, ConfigMixin):
         encoder_hidden_states: torch.Tensor,
         return_dict: bool = True,
     ) -> Union[UNet2DConditionOutput, Tuple]:
-        """r
+        r"""
         Args:
             sample (`torch.FloatTensor`): (batch, channel, height, width) noisy inputs tensor
             timestep (`torch.FloatTensor` or `float` or `int`): (batch) timesteps