Update mypy issue reference to duplicated original issue (#11320)

See python/mypy#13975 (comment)

Author: Avasam

stubs/pika/@tests/stubtest_allowlist.txt

@@ -3,7 +3,7 @@ pika.compat.StringIO.seek
 pika.compat.StringIO.truncate
 
 # Stubtest doesn't understand that a property alias is also read-only.
-# https://github.com/python/mypy/issues/13975
+# https://github.com/python/mypy/issues/6700
 pika(\.adapters)?(\.blocking_connection)?\.BlockingConnection\.basic_nack
 pika(\.adapters)?(\.blocking_connection)?\.BlockingConnection\.consumer_cancel_notify
 pika(\.adapters)?(\.blocking_connection)?\.BlockingConnection\.exchange_exchange_bindings