Auto merge of #8551 - CPerezz:manifest_edition_nit, r=Eh2406

Revert Cargo Book changes for default edition

On #8543 it was introduced a change on the Cargo Book docs
where it was incorrectly stated that the default edition for
the Cargo Manifest was `2018` when indeed, as @ehuss correctly stated
in https://github.com/rust-lang/cargo/pull/8543#issuecomment-663769077
it is not.

Therefore, as @Eh2406 asked for, I made the PR reverting this changes
and leaving the Cargo Book docs as they were before.