[fix] an off by one error in the upper bound of rfind()

Author: nickomodee

src/partial_json_parser/core/myelin.py

@@ -120,7 +120,7 @@ def truncate_before_last_key_start(container_start: int, last_string_end: int, s
             # { "k
             return json_string[: stack[-1][0] + 1], join_closing_tokens(stack)
 
-        last_comma = json_string.rfind(",", max(stack[-1][0], last_string_end) + 1, last_string_start - 1)
+        last_comma = json_string.rfind(",", max(stack[-1][0], last_string_end) + 1, last_string_start)
         if last_comma != -1:
             # { "key": "v", "k
             # { "key": 123, "k