rustc lint listing code snippets don't have any indentation

[camelid]

It used to (on stable and beta) look like this:

But on nightly it looks like:

The source code for the snippet is:

    /// ### Example
    ///
    /// ```rust
    /// if let _ = 123 {
    ///     println!("always runs!");
    /// }
    /// ```

This is true for other code snippets in the lint listing, though the rest of the book (and other books) don't have the issue.

[camelid]

cc @ehuss since you're the only one who's touched lint-docs since September

[camelid]

Assigning P-high and removing I-prioritize as discussed in the prioritization working group.