Update description.

README.md

@@ -18,18 +18,19 @@ For example, if you're using Apple Silicon Mac and booting Ubuntu Server for ARM
 
 First, clone this repository.
 
-In the repository directory, download `ubuntu-20.04.1-live-server-arm64.iso` from [Ubuntu Server for ARM](https://ubuntu.com/download/server/arm).
+In the repository directory, download an iso file from [Ubuntu Server for ARM](https://ubuntu.com/download/server/arm). The following URL and file name of the iso file includes version may vary, so please check it at the download page.
 
 ```sh
-curl -O 'https://cdimage.ubuntu.com/releases/20.04/release/ubuntu-20.04.1-live-server-arm64.iso'
+curl -O 'https://cdimage.ubuntu.com/releases/20.04/release/ubuntu-20.04.3-live-server-arm64.iso'
 ```
 
 Then, mount downloaded iso file. You need to use `mount` instead of double-click the iso file. Extract Linux kernel (and ungzip,) and ramdisk.
 
 ```sh
+# Create a mounting point.
 mkdir -p ubuntu
 # Attach and mount the iso file
-hdiutil attach -nomount ubuntu-20.04.1-live-server-arm64.iso
+hdiutil attach -nomount ubuntu-20.04.3-live-server-arm64.iso
 mount -t cd9660 /dev/disk4 ubuntu # disk4 may vary depends on your environment, see output of `hdiutil`
 # Copy linux kernel and ungzip
 cp ubuntu/casper/vmlinuz vmlinux.gz
@@ -39,6 +40,8 @@ cp ubuntu/casper/initrd ./
 # Unmount and detach the iso file
 umount ubuntu
 hdiutil detach disk4 # Same here.
+# Remove the mounting point.
+rm -rf ubuntu
 ```
 
 Build TinyLinux then boot.
@@ -51,5 +54,5 @@ make
   --vmlinux vmlinux \
   --initrd initrd \
   --commandline "console=hvc0 root=/dev/vda1" \
-  --image ubuntu-20.04.1-live-server-arm64.iso
+  --image ubuntu-20.04.3-live-server-arm64.iso
 ```