Fix broken IdentFragment link in format_ident doc

    warning: unresolved link to `IdentFragment`
      --> src/format.rs:13:16
       |
    13 | /// * `{}` ⇒ [`IdentFragment`]
       |                ^^^^^^^^^^^^^ no item named `IdentFragment` in scope
       |
       = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
       = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

    warning: unresolved link to `IdentFragment`
      --> src/format.rs:25:45
       |
    25 | /// Unlike `format!`, this macro uses the [`IdentFragment`] formatting trait by
       |                                             ^^^^^^^^^^^^^ no item named `IdentFragment` in scope
       |
       = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

src/format.rs

@@ -29,6 +29,7 @@
 ///    unsigned integers and strings.
 /// * [`Ident`] arguments will have their `r#` prefixes stripped, if present.
 ///
+/// [`IdentFragment`]: crate::IdentFragment
 /// [`Ident`]: proc_macro2::Ident
 ///
 /// <br>