Update the wait API example

Author: rajkan01

platform/mbed_wait_api.h

@@ -40,15 +40,22 @@ extern "C" {
  * Example:
  * @code
  * #include "mbed.h"
- *
- * DigitalOut heartbeat(LED1);
- *
+ * 
+ * // Blinking rate in milliseconds
+ * #define BLINKING_RATE_MS   500
+ * DigitalOut  led(LED2);
+ * InterruptIn button(SW2);
+ * 
+ * void blink_led() {
+ *     led = 1;
+ *     wait_us(BLINKING_RATE_MS * 1000);
+ *     led = 0;
+ * }
+ * 
  * int main() {
- *     while (1) {
- *         heartbeat = 1;
- *         wait_us(500);
- *         heartbeat = 0;
- *         wait_us(500);
+ *     button.fall(&blink_led);
+ *     while(1) {
+ *         // Do nothing
  *     }
  * }
  * @endcode