Fix empty-set symbol in comments

Author: dtolnay

proc_macro/src/lib.rs

@@ -811,7 +811,7 @@ pub enum Delimiter {
     /// `[ ... ]`
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     Bracket,
-    /// `Ø ... Ø`
+    /// `∅ ... ∅`
     /// An invisible delimiter, that may, for example, appear around tokens coming from a
     /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
     /// `$var * 3` where `$var` is `1 + 2`.