Fix doc error with `rust-toolchain.toml` custom TC
Fix an issue where the documentation erroneously states that
`rust-toolchain.toml` config files can't specify custom toolchain names.
This language seems to be very old, ultimately inherited from the
original commit that added this feature, and is no longer true:
https://github.com/rust-lang/rustup/commit/107d8e5f1ab83ce13cb33a7b4ca0f58198285ee8#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5R330
