multi-byte utf-8 characters in doc comments break docify

Code sample:

```rust
/// Example struct holding the most recently set [`u32`] and the second
/// most recently set [`u32`] (if one existed).
pub struct Foo;
```

Fixes when word wrapping is tweaked to:

```rust
/// Example struct holding the most recently set [`u32`] and the
/// second most recently set [`u32`] (if one existed).
pub struct Foo;
```

cc @liamaharon