update dd command for bootable arch installer

active/os_arch/arch-base.md

@@ -70,10 +70,15 @@ Follow most of the instructions here: <https://wiki.archlinux.org/title/Installa
     ```bash
     gpg --auto-key-locate clear,wkd -v --locate-external-key pierre@archlinux.org
     gpg --keyserver-options auto-key-retrieve --verify archlinux-...
+    gpg --verify signature_file.sig archlinux.iso
     ```
 
 3. Create a bootable ISO <https://wiki.archlinux.org/title/USB_flash_installation_medium>
 
+    ```bash
+    dd bs=4M if=path/to/archlinux-version-x86_64.iso of=/dev/disk/by-id/usb-My_flash_drive conv=fsync oflag=direct status=progress
+    ```
+
     1. If you are booting into a VM, create an ISO with installation files so you don't have to
        copy-paste: