Remove trailing comma from json

Change-Id: I2df9b3342b5d166606c3bc13569896dfeac20fe4
Reviewed-on: https://dart-review.googlesource.com/c/85960
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Keerti Parthasarathy <[email protected]>

Author: keertip

runtime/vm/service/service.md

@@ -150,7 +150,7 @@ a parameter:
   "jsonrpc": "2.0",
   "method": "streamListen",
   "params": {
-    "streamId": "GC",
+    "streamId": "GC"
   },
   "id": "2"
 }