Fix unbound error (pallets#2039)

Author: dongweiming

flask/app.py

@@ -519,7 +519,7 @@ def __init__(self, import_name, static_path=None, static_url_path=None,
         #:        def to_python(self, value):
         #:            return value.split(',')
         #:        def to_url(https://github.com/self,%20values):
-        #:            return ','.join(BaseConverter.to_url(https://github.com/value)
+        #:            return ','.join(super(ListConverter, self).to_url(https://github.com/value)
         #:                            for value in values)
         #:
         #:    app = Flask(__name__)