Fix link from custom_punctuation to Span
diff --git a/src/custom_punctuation.rs b/src/custom_punctuation.rs
index 3380f88..8d7d444 100644
--- a/src/custom_punctuation.rs
+++ b/src/custom_punctuation.rs
@@ -25,7 +25,7 @@
/// [Peeking]: parse/struct.ParseBuffer.html#method.peek
/// [Parsing]: parse/struct.ParseBuffer.html#method.parse
/// [Printing]: https://docs.rs/quote/0.6/quote/trait.ToTokens.html
-/// [`Span`]: struct.Span.html
+/// [`Span`]: https://docs.rs/proc-macro2/0.4/proc_macro2/struct.Span.html
///
/// # Example
///