fix a small typo in pipeline_ddpm.py (#948)

one small typo in pipeline_ddpm.py

just a small typo in one comment

Author: chenguolin

src/diffusers/pipelines/ddpm/pipeline_ddpm.py

@@ -79,7 +79,7 @@ def __call__(
             # 1. predict noise model_output
             model_output = self.unet(image, t).sample
 
-            # 2. compute previous image: x_t -> t_t-1
+            # 2. compute previous image: x_t -> x_t-1
             image = self.scheduler.step(model_output, t, image, generator=generator).prev_sample
 
         image = (image / 2 + 0.5).clamp(0, 1)