Fix incorrect print format in json.c

Oid is unsigned, so %u needs to be used and not %d.  The code path
involved here is not normally reachable, so no backpatch is done.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20200728015523.GA27308@telsasoft.com

src/backend/utils/adt/json.c

@@ -458,7 +458,7 @@ JsonEncodeDateTime(char *buf, Datum value, Oid typid, const int *tzp)
 			}
 			break;
 		default:
-			elog(ERROR, "unknown jsonb value datetime type oid %d", typid);
+			elog(ERROR, "unknown jsonb value datetime type oid %u", typid);
 			return NULL;
 	}