Update main.cpp

Make sure printf gets '\n'

Author: sg-

libraries/tests/mbed/serial_complete/main.cpp

@@ -9,7 +9,7 @@ int main() {
     MBED_HOSTTEST_DESCRIPTION(Serial Complete);
     MBED_HOSTTEST_START("MBED_39");
 
-    pc->printf("123456789");
+    pc->printf("123456789\n");
 
     while (1) {
         deepsleep();