Fix typo in rate limiting requests per second

[devlikepro]

The default value for RACK_ATTACK_LIMIT is 3000, so the documentation should state either 3000 rpm or 60 rps.
Since rate limits are usually expressed per second, it makes more sense to describe it as 60 rps instead of 60 rpm.

throttle('req/ip', limit: ENV.fetch('RACK_ATTACK_LIMIT', '3000').to_i, period: 1.minute, &:ip)