Fix E0_SERIAL_RX_PIN typo (#15841)

Marlin/src/pins/lpc1768/pins_RAMPS_RE_ARM.h

@@ -152,7 +152,7 @@
   #ifndef E0_SERIAL_TX_PIN
     #define E0_SERIAL_TX_PIN P2_08
   #endif
-  #ifndef E0_SESIAL_RX_PIN
+  #ifndef E0_SERIAL_RX_PIN
     #define E0_SERIAL_RX_PIN P2_08
   #endif