readme: ATmega1284 has 16k of SRAM (#13421)

README.md

@@ -30,7 +30,7 @@ Marlin 2.0 introduces a layer of abstraction so that all the existing high-level
 
   name|processor|speed|flash|sram|logic|fpu
   ----|---------|-----|-----|----|-----|---
-  [Arduino AVR](https://www.arduino.cc/)|ATmega, ATTiny, etc.|16-20MHz|64-256k|2-8k|5V|no
+  [Arduino AVR](https://www.arduino.cc/)|ATmega, ATTiny, etc.|16-20MHz|64-256k|2-16k|5V|no
   [Teensy++ 2.0](http://www.microchip.com/wwwproducts/en/AT90USB1286)|[AT90USB1286](http://www.microchip.com/wwwproducts/en/AT90USB1286)|16MHz|128k|8k|5V|no
   [Due](https://www.arduino.cc/en/Guide/ArduinoDue), [RAMPS-FD](http://www.reprap.org/wiki/RAMPS-FD), etc.|[SAM3X8E ARM-Cortex M3](http://www.microchip.com/wwwproducts/en/ATsam3x8e)|84MHz|512k|64+32k|3.3V|no
   [Re-ARM](https://www.kickstarter.com/projects/1245051645/re-arm-for-ramps-simple-32-bit-upgrade)|[LPC1768 ARM-Cortex M3](http://www.nxp.com/products/microcontrollers-and-processors/arm-based-processors-and-mcus/lpc-cortex-m-mcus/lpc1700-cortex-m3/512kb-flash-64kb-sram-ethernet-usb-lqfp100-package:LPC1768FBD100)|100MHz|512k|32+16+16k|3.3-5V|no