Fix default edition spec in Cargo Book

The Manifest section states on the example that the
default edition is 2018, but the lines of text avobe
state that the default is `2015` which is not correct.

Closes #8541

Author: CPerezz

src/doc/src/reference/manifest.md

@@ -112,7 +112,7 @@ brackets at the end of each author.
 
 You can opt in to a specific [Rust Edition] for your package with the
 `edition` key in `Cargo.toml`. If you don't specify the edition, it will
-default to 2015.
+default to 2018.
 
 ```toml
 [package]