Fix type in Dockerfile

Author: uraimo

Dockerfiles/Dockerfile.aarch64.ubuntu20.04

@@ -1,4 +1,4 @@
-FROM --platform=linux/arm64 rm64v8/ubuntu:20.04
+FROM --platform=linux/arm64 arm64v8/ubuntu:20.04
 
 COPY ./run-on-arch-install.sh /root/run-on-arch-install.sh
 RUN chmod +x /root/run-on-arch-install.sh && /root/run-on-arch-install.sh