ndk: Make examples in ndk::midi compilable

Author: paxbun

ndk/src/midi/mod.rs

@@ -28,7 +28,9 @@ pub enum MidiOpcode {
         /// The length of the received message. The caller should limit the passed `buffer` slice to
         /// this length after [`MidiOutputPort::receive()`] returns.
         /// ```no_run
-        /// let output_port: MidiOutputPort = ...;
+        /// use ndk::midi::MidiOutputPort;
+        ///
+        /// let output_port: MidiOutputPort = todo!();
         /// let mut buffer = [0u8; 128];
         /// if let Ok(MidiOpcode::Data { length, .. }) = output_port.receive(&mut buffer) {
         ///     let buffer = &buffer[..length];