Resolve doc_overindented_list_items clippy lint

    warning: doc list item overindented
      --> src/format.rs:29:5
       |
    29 | ///    unsigned integers and strings.
       |     ^^^ help: try using `  ` (2 spaces)
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items
       = note: `-W clippy::doc-overindented-list-items` implied by `-W clippy::all`
       = help: to override `-W clippy::all` add `#[allow(clippy::doc_overindented_list_items)]`

Author: dtolnay

src/format.rs

@@ -26,7 +26,7 @@
 /// default. This trait is like `Display`, with a few differences:
 ///
 /// * `IdentFragment` is only implemented for a limited set of types, such as
-///    unsigned integers and strings.
+///   unsigned integers and strings.
 /// * [`Ident`] arguments will have their `r#` prefixes stripped, if present.
 ///
 /// [`IdentFragment`]: crate::IdentFragment