Update IoPin documentation

Author: Morgan Roff

src/digital.rs

@@ -177,8 +177,8 @@ pub trait InputPin {
 
 /// Single pin that can switch from input to output mode, and vice-versa.
 ///
-/// Example use (assumes the `Error` type is the same for `InputPin`, and
-/// `OutputPin`):
+/// Example use (assumes the `Error` type is the same for the `IoPin`,
+/// `InputPin`, and `OutputPin`):
 ///
 /// ```
 /// use core::time::Duration;