remove multilingual field from book.toml

[mysteriouslyseeing]

When running mdbook serve book/ as shown in book/README.md, mdbook returns an error:

ERROR Invalid configuration file
	Caused by: TOML parse error at line 3, column 1
  |
3 | multilingual = false
  | ^^^^^^^^^^^^
unknown field `multilingual`, expected one of `title`, `authors`, `description`, `src`, `language`, `text-direction`

Apparently this field never did anything and was removed here.

It's a one-line fix to remove the field from the book/book.toml file.