io: fix typo

tokio/src/io/interest.rs

@@ -131,7 +131,7 @@ impl Interest {
     /// const RW_INTEREST: Interest = Interest::READABLE.add(Interest::WRITABLE);
     ///
     /// let w_interest = RW_INTEREST.remove(Interest::READABLE).unwrap();
-    /// assert!(!w_interest.is_is_readable());
+    /// assert!(!w_interest.is_readable());
     /// assert!(w_interest.is_writable());
     ///
     /// // Removing all interests from the set returns `None`.