Fix (broken) link to Install ReArm tutorial (#12823)

README.md

 
 ## Building Marlin 2.0
 
-To build Marlin 2.0 you'll need [Arduino IDE 1.9](https://www.arduino.cc/en/main/software) or [PlatformIO](http://docs.platformio.org/en/latest/ide.html#platformio-ide). We've posted detailed instructions on how to [build Marlin 2.0 for ARM](http://marlinfw.org/docs/basics/install_arm.html).
+To build Marlin 2.0 you'll need [Arduino IDE 1.9](https://www.arduino.cc/en/main/software) or [PlatformIO](http://docs.platformio.org/en/latest/ide.html#platformio-ide). We've posted detailed instructions on how to [build Marlin 2.0 for ARM](http://marlinfw.org/docs/basics/install_rearm.html).
 
 ## Hardware Abstraction Layer (HAL)