install: Add -U to genfstab examples

Author: wezm

_docs/installation/install.md

@@ -250,14 +250,14 @@ The installation scripts come with a `fstab` generator. You can invoke
 it like:
 
 ```
-# genfstab / >> /etc/fstab
+# genfstab -U / >> /etc/fstab
 ```
 
 It is also possible to invoke it from the outside of the system, e.g.
 like:
 
 ```
-# genfstab /media/root >> /media/root/etc/fstab
+# genfstab -U /media/root >> /media/root/etc/fstab
 ```
 
 You might want to manually edit the generated `fstab` to remove useless