Update docstring in chat_completion API resource (#527)

Fixes a link to our docs.

openai/api_resources/chat_completion.py

@@ -14,7 +14,7 @@ def create(cls, *args, **kwargs):
         """
         Creates a new chat completion for the provided messages and parameters.
 
-        See https://platform.openai.com/docs/api-reference/chat-completions/create
+        See https://platform.openai.com/docs/api-reference/chat/create
         for a list of valid parameters.
         """
         start = time.time()
@@ -34,7 +34,7 @@ async def acreate(cls, *args, **kwargs):
         """
         Creates a new chat completion for the provided messages and parameters.
 
-        See https://platform.openai.com/docs/api-reference/chat-completions/create
+        See https://platform.openai.com/docs/api-reference/chat/create
         for a list of valid parameters.
         """
         start = time.time()