Merge pull request #262 from dtolnay/syn2

Update syn 1.0 link to syn 2.0

src/lib.rs

@@ -428,7 +428,7 @@ pub mod spanned;
 /// appears suffixed as integer literals by interpolating them as [`syn::Index`]
 /// instead.
 ///
-/// [`syn::Index`]: https://docs.rs/syn/1.0/syn/struct.Index.html
+/// [`syn::Index`]: https://docs.rs/syn/2.0/syn/struct.Index.html
 ///
 /// ```compile_fail
 /// let i = 0usize..self.fields.len();