doc: add symbol comma into chatml.md (#329)

Author: liuliuOD

chatml.md

@@ -59,7 +59,7 @@ We do not currently allow autocompleting of partial messages,
 ```
 [
  {"token": "<|im_start|>"},
- "system\nPlease autocomplete the user's message."
+ "system\nPlease autocomplete the user's message.",
  {"token": "<|im_end|>"}, "\n", {"token": "<|im_start|>"},
  "user\nThis morning I decided to eat a giant"
 ]
@@ -87,7 +87,7 @@ Comment allez-vous?
 {{user input here}}<|im_end|>
 ```
 If adding instructions in the `system` message doesn't work, you can
-also try putting them into a `user` message.  (In the near future, we
+also try putting them into a `user` message. (In the near future, we
 will train our models to be much more steerable via the system
 message. But to date, we have trained only on a few system messages,
 so the models pay much more attention to user examples.)