FIX loop function

Arduino15/packages/arduino/hardware/avr/1.6.21/cores/arduino/main.cpp:46: undefined reference to `loop'
collect2: error: ld returned 1 exit status
exit status 1
Error compiling for board Arduino/Genuino Uno.

a/arduino.ino

@@ -1,4 +1,12 @@
 void setup() {
+  // put your setup code here, to run once:
+  // initialize serial communication at 9600 bits per second:
   Serial.begin(9600);
+  //print message serial monitor
   Serial.println("Hello world!");
 }
+
+void loop() {
+  // put your main code here, to run repeatedly:
+
+}