Empty kwargs when implementing the example code from the docs

I have posted this issue to StackOverflow:

https://stackoverflow.com/questions/67574137/flask-apispec-not-populating-kwargs-with-values-from-get-query-implementation-o

The following simple program below, which is nearly identical to the example provided in the docs, does not work:

```
import flask
from flask_apispec import use_kwargs
from webargs import fields

app = flask.Flask(__name__)

@app.route('/pets', methods=["GET"])
@use_kwargs({'species': fields.Str()})
def list_pets(**kwargs):
assert False, kwargs # NO DATA HERE

if __name__ == "__main__":
app.run(host="0.0.0.0", port=5000)
```

The `kwargs` field is empty.

This is with flask v1.1.2.

Start with the provided program and the @use_kwargs entry point, running the example with Flask 1.1.2 and a GET request containing the species query parameter. Compare its behavior with the documentation example; done means the parsed query value is available in kwargs instead of an empty dictionary.

Written by the indexing model from the issue text.