Typo mentioning stdout instead of stderr in the docs for eprint/eprintln

[HomelikeBrick42]

Location

https://github.com/rust-lang/rust/blob/master/library/std/src/macros.rs#L192-L193
https://github.com/rust-lang/rust/blob/master/library/std/src/macros.rs#L192-L193

Summary

These macros print to stderr, so it should probably mention stderr instead of stdout