Documentation: fix that default_midi_port now gives an optional

Author: jcelerier

book/src/midi-1-in.md

@@ -24,7 +24,8 @@ libremidi::midi_in midi{
 // The argument is a libremidi::input_port gotten from a libremidi::observer. 
 midi.open_port(/* a port */);
 // Alternatively, to get the default port for the system: 
-midi.open_port(libremidi::midi1::in_default_port());
+if(auto port = libremidi::midi1::in_default_port())
+  midi.open_port(*port);
 
 // Note that only one port can be open at a given time on a midi_in or midi_out object.
 

book/src/midi-1-out.md

@@ -5,7 +5,8 @@
 libremidi::midi_out midi;
 
 // Open a given midi port. Same as for input:
-midi.open_port(libremidi::midi2::out_default_port());
+if(auto port = libremidi::midi1::out_default_port())
+  midi.open_port(*port);
 
 // Option A: send fixed amount of bytes for most basic cases
 midi.send_message(144, 110, 40); // Overloads exist for 1, 2, 3 bytes

book/src/midi-2-out.md

@@ -5,7 +5,8 @@
 libremidi::midi_out midi;
 
 // Open a given midi port. Same as for input:
-midi.open_port(libremidi::midi2::out_default_port());
+if(auto port = libremidi::midi2::out_default_port())
+  midi.open_port(*port);
 
 // Option A: send fixed amount of bytes for most basic cases
 midi.send_ump(A, B, C, D); // Overloads exist for 1, 2, 3, 4 uint32_t