Ctrl + Enter playpen shortcut

[vvanpo]

The rust-by-example documentation invites us to use "ctrl + enter" to run playpen code, but this doesn't work (at least, not for macOS in either Chrome nor Safari).

[rnitta]

It doesn't seem to be issue of mdbook.

// or if you prefer to use your keyboard, you can use the "Ctrl + Enter" shortcut

It may be the dead comment of rust by example.

Rust-by-example used to be built with gitbook, but currently using mdbook.
At that time, the command Ctrl-Enter would work, but it seems that the feature had lost in the migration from gitbook to mdbook.

rust-lang/rust-by-example@f39d818

Though correcting the comment of rust-by-exmaple is reasonable,
there is the way to fix the problem by adding command to default mdbook's theme; #1066

:D