Update book.toml

This specifies the new mdBook syntax (properties under `[book]` table).

It also adds the `editor = "ace"` config, which will fix #963. Once
rust-lang/mdBook#515 is merged, this
won't be necessary.

Author: jridgewell

book.toml

@@ -1,6 +1,8 @@
+[book]
 title = "Rust By Example"
 description = "A description"
 author = "The Rust Community"
 
 [output.html.playpen]
 editable = true
+editor = "ace"