Update serial_communication.md

Added NUCLEO-F401RE pin map for clarification

Author: AnotherButler

docs/getting_started/serial_communication.md

@@ -95,6 +95,8 @@ int main() {
 
 <span class="tips">**Note:** This example only works if LED1 is on the Pwm pin of the board you are using, such as the NUCLEO-F401RE. </span>
 
+<span class="images">![](NUCLEO-F401RE.PNG)<span>The pin map of the NUCLEO-F401RE shows LED1 on the Pwm pin.</span></span>
+
 ```c
 #include "mbed.h"