Update section-6.md: fly.io seems to offer a free trial plan

[pkoivisto]

It appears fly.io offers a free trial plan, even though this is not advertised on their site. The trial plan is what my account ended up showing after following a quick start guide to launching an app on fly.io. No payment details were asked at any point of the process.

[qzuw]

Fly.io was tried in tsoha when Heroku ended their free tier. Personally I got it working for me without credit card, but some students reported it required cc for them, so this might be worth double checking.

Also, stylistically "seems to something" may not be best phraseology for teaching material. "Might allow trial without cc" or something similar may be better.