docs: sample code fix typo

Author: evesquare

README.md

@@ -179,7 +179,7 @@ def get_clip(clip_id):
     return response.json()
 
 def generate_whole_song(clip_id):
-    payloyd = {"clip_id": clip_id}
+    payload = {"clip_id": clip_id}
     url = f"{base_url}/api/concat"
     response = requests.post(url, json=payload)
     return response.json()