Fix default stack size in stdlib docs

[Kobzol]

It was changed from 2 MiB to 8 MiB three years ago (#55617). The size is now 8 MiB (

rust/compiler/rustc_interface/src/util.rs

Line 134 in 8ed1d4a

const STACK_SIZE: usize = 8 * 1024 * 1024;

).

[rustbot]

r? @joshtriplett

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[Kobzol]

Nevermind, I mixed up the stdlib and the rustc default.