Switch sample to use SerialUSB

I commented out the UART1 test and switched to using SerialUSB instead.
This work with my setup when I use a full-speed hub but not via high
speed USB. I am starting to get suspicious that it might have something
to do with my USB-C cable so I am pushing up this change for others to
try.

Author: adamgreen

libraries/ThreadMRI/examples/ThreadMRI/ThreadMRI.ino

@@ -5,7 +5,8 @@
 
 #include <ThreadMRI.h>
 
-ThreadMRI g_debugger(Serial1, USART1_IRQn, 115200, true);
+//ThreadMRI g_debugger(Serial1, USART1_IRQn, 115200, true);
+ThreadMRI g_debugger(Serial, OTG_HS_IRQn, 115200, true);
 
 extern "C" void testContext(void);