Changed sample[0] to images[0] (#3304)

A pipeline object stores the results in `images` not in `sample`.
Current code blocks don't work.

src/diffusers/pipelines/stable_diffusion/README.md

@@ -61,7 +61,7 @@ pipe = StableDiffusionPipeline.from_pretrained("runwayml/stable-diffusion-v1-5")
 pipe = pipe.to("cuda")
 
 prompt = "a photo of an astronaut riding a horse on mars"
-image = pipe(prompt).sample[0]  
+image = pipe(prompt).images[0]  
     
 image.save("astronaut_rides_horse.png")
 ```
@@ -80,7 +80,7 @@ pipe = StableDiffusionPipeline.from_pretrained(
 ).to("cuda")
 
 prompt = "a photo of an astronaut riding a horse on mars"
-image = pipe(prompt).sample[0]  
+image = pipe(prompt).images[0]  
     
 image.save("astronaut_rides_horse.png")
 ```
@@ -99,7 +99,7 @@ pipe = StableDiffusionPipeline.from_pretrained(
 ).to("cuda")
 
 prompt = "a photo of an astronaut riding a horse on mars"
-image = pipe(prompt).sample[0]  
+image = pipe(prompt).images[0]  
     
 image.save("astronaut_rides_horse.png")
 ```