changed author to authors in book.toml

Thanks to Gábor Szabó (szabgab@github) for this patch.

Author: BartMassey

mdbook/book.toml

@@ -1,7 +1,7 @@
 [book]
 title = "Discovery"
 description = "Discover the world of microcontrollers through Rust with the BB2 micro:bit v2"
-author = "Rust Embedded Resources Team"
+authors = "Rust Embedded Resources Team"
 language = "en"
 
 [output.html]