AGCM4 link in README.md (#15461)

README.md

   ----|---------|-----|-----|----|-----|---
   [STEVAL-3DP001V1](http://www.st.com/en/evaluation-tools/steval-3dp001v1.html)|[STM32F401VE Arm-Cortex M4](http://www.st.com/en/microcontrollers/stm32f401ve.html)|84MHz|512k|64+32k|3.3-5V|yes
   [Smoothieboard](http://reprap.org/wiki/Smoothieboard)|LPC1769 ARM-Cortex M3|120MHz|512k|64k|3.3-5V|no
-  [Adafruit Grand Central M4](https://www.adafruit.com/product/4064)|ARM-Cortex M4|120MHz|1M|256k|3.3V|yes
+  [Adafruit Grand Central M4](https://www.adafruit.com/product/4064)|[SAMD51P20A ARM-Cortex M4](https://www.microchip.com/wwwproducts/en/ATSAMD51P20A)|120MHz|1M|256k|3.3V|yes
 
 ## Submitting Patches