Fix broken link to format_ident macro documentation

    warning: unresolved link to `format_ident`
      --> src/ident_fragment.rs:10:11
       |
    10 | /// See [`format_ident!`] for more information.
       |           ^^^^^^^^^^^^^ no item named `format_ident` in scope
       |
       = note: `macro_rules` named `format_ident` exists in this crate, but it is not in scope at this link's location
       = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

src/ident_fragment.rs

@@ -8,6 +8,8 @@ use std::borrow::Cow;
 /// stripped, if present.
 ///
 /// See [`format_ident!`] for more information.
+///
+/// [`format_ident!`]: crate::format_ident
 pub trait IdentFragment {
     /// Format this value as an identifier fragment.
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result;