Modify the file main.cpp for M487

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+# Getting started with LED and buzzer on mbed OS
+
+
+### Import NuMaker-mbed-GPIO-button-buzzer-rgbled from on-line IDE
+1. Please choose Nuvoton NuMaker-PFM-XXX as your target platform.
+2. Please press the left-up icon "New", then choose "NuMaker GPIO button with LED and buzzer" from the template list.
+3. Your NuMaker-mbed-GPIO-button-buzzer-rgbled program is existed.
+
+#### Now compile
+Please press compile icon.
+
+#### Burn Code & Execute
+1. Connect the board NuMaker-PFM-XXX with your PC by USB cable, then there will be one "mbed" disk.
+2. Copy the built binary file into "mbed" disk on you PC.
+3. Press device's reset button to execute, press the SW1 button to turn on blue LED and green LED, 
+   press the SW2 button to turn on buzzer.