docs/installation-usb: add more flags to dd

oflag=direct is faster for writing because it by-passes the page cache
status=progress shows speed and progress

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manual
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nixos/doc/manual/installation/installing-usb.section.md
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4. Then use the `dd` utility to write the image to the USB flash drive.
```ShellSession
-
sudo dd if=<path-to-image> of=/dev/sdX bs=4M conv=fsync
+
sudo dd bs=4M conv=fsync oflag=direct status=progress if=<path-to-image> of=/dev/sdX
```
## Creating bootable USB flash drive from a Terminal on macOS {#sec-booting-from-usb-macos}