[docs] [typo] Small typo in async_std::stream docs

[leklachu]

Just a small typo in the docs for stream,

Streams do not have to be finite. As an example, an repeat stream is an infinite stream

https://docs.rs/async-std/1.6.3/async_std/stream/index.html#infinity

Looks like it's autoconversion from std::iter
https://doc.rust-lang.org/std/iter/index.html#infinity

[surechen]

Hello, I noticed this problem when browsing the issues and I agree with your opinion. I think I can submit a PR to fix this problem.

[Fishrock123]

Fixed by #926