docs: improve upstream md-book code snippets display

[iqdecay]

• We now want all code that goes in the book to be tested (and put in the examples directory), so all code in The Book will soon be put there using the {{include path/to/file}} tag of md-book.

Problem: the formatting isn 't very nice, there's extra identation displayed because the code is inside a test function.

Fixing rust-lang/mdBook#1601 would solve this! It would also be of benefit for all books!

Tagging @mitchmindtree for tooling consideration 😄