fix: make the rate limit example runnable

[iklobato]

Small fix on top of your branch for iklobato#48. Only touches examples/19_rate_limit.py.

I tried to run the example and it did not start. Three problems, each one enough to break it:

1. from lightapi import ... LightAPI -> the class is LightApi, so the import fails right away:

ImportError: cannot import name 'LightAPI' from 'lightapi'. Did you mean: 'LightApi'?

2. rate_limit = {"request": 3, ...} -> the middleware reads requests, so every call to /books returns 500:

GET /books HTTP/1.1" 500 Internal Server Error
KeyError: 'requests'

3. app.run(..., reload=True) -> uvicorn needs an import string for reload, so it just prints a warning and exits:

WARNING:  You must pass the application as an import string to enable 'reload' or 'workers'.

Changed it to debug=True, which is what the other 18 examples use.

I ran the example after the change and called the endpoint:

req 1: HTTP/1.1 200 OK | x-ratelimit-remaining: 2
req 2: HTTP/1.1 200 OK | x-ratelimit-remaining: 1
req 3: HTTP/1.1 200 OK | x-ratelimit-remaining: 0
req 4: HTTP/1.1 429 Too Many Requests | x-ratelimit-remaining: 0

The middleware itself is unchanged. Your 4 tests still pass and so does the full suite (331 passed).