[Docs] fix docstring for issue #709 (#710)

fix docstring

fixes #709

src/diffusers/models/unet_2d.py

@@ -170,7 +170,7 @@ class UNet2DModel(ModelMixin, ConfigMixin):
         timestep: Union[torch.Tensor, float, int],
         return_dict: bool = True,
     ) -> Union[UNet2DOutput, Tuple]:
-        """r
+        r"""
         Args:
             sample (`torch.FloatTensor`): (batch, channel, height, width) noisy inputs tensor
             timestep (`torch.FloatTensor` or `float` or `int): (batch) timesteps

src/diffusers/models/unet_2d_condition_flax.py

@@ -215,7 +215,7 @@ class FlaxUNet2DConditionModel(nn.Module, FlaxModelMixin, ConfigMixin):
         return_dict: bool = True,
         train: bool = False,
     ) -> Union[FlaxUNet2DConditionOutput, Tuple]:
-        """r
+        r"""
         Args:
             sample (`jnp.ndarray`): (channel, height, width) noisy inputs tensor
             timestep (`jnp.ndarray` or `float` or `int`): timesteps