docs: fix typo

[GarmashAlex]

fixed a minor typo
from substutition to substitution

[rustbot]

r? @wesleywiser

rustbot has assigned @wesleywiser.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol

[jieyouxu]

Please don't send trivial typo fixes for rustc-dev-guide against this repo unless it accompanies a compiler/library/bootstrap/rustdoc change too, as it increase chance of josh merge conflicts. Instead, please send it against https://github.com/rust-lang/rustc-dev-guide/. Thanks for the PR.