Update book.toml fix the authors field

See https://rust-lang.github.io/mdBook/format/configuration/general.html#general-metadata

Author: szabgab

Diff for: book.toml

@@ -1,7 +1,7 @@
 [book]
 
 title = "The Rust Reference"
-author = "The Rust Project Developers"
+authors = ["The Rust Project Developers"]
 
 [output.html]
 additional-css = ["theme/reference.css"]