rename the author field to be authors in book.toml (#15362)

See the [mdbook
documentation](https://rust-lang.github.io/mdBook/format/configuration/general.html)
Apparently the `author` was field was [incorrect
documentation](rust-lang/mdBook#2623)

Author: weihanglo

src/doc/book.toml

@@ -1,6 +1,6 @@
 [book]
 title = "The Cargo Book"
-author = "Alex Crichton, Steve Klabnik and Carol Nichols, with contributions from the Rust community"
+authors = ["Alex Crichton", "Steve Klabnik", "Carol Nichols", "with contributions from the Rust community"]
 
 [output.html]
 smart-punctuation = true # Enable smart-punctuation feature for more than quotes.