Correct ESP32 CPU speed in README (#16472)

README.md

@@ -46,7 +46,7 @@ Marlin 2.0 introduces a layer of abstraction so that all the existing high-level
 
   board|processor|speed|flash|sram|logic|fpu
   ----|---------|-----|-----|----|-----|---
-  [ESP32](https://www.espressif.com/en/products/hardware/esp32/overview)|Tensilica Xtensa LX6|320MHz|---|---|3.3V|---
+  [ESP32](https://www.espressif.com/en/products/hardware/esp32/overview)|Tensilica Xtensa LX6|240MHz|---|---|3.3V|---
 
   #### LPC1768 / LPC1769