Ubuntu/Debian: use apt instead of dpkg

In general apt is the preferred way to interact with packages, and I
think in this case there is no need/advantage to use dpkg.

Closes #88

Signed-off-by: Richard Laager <rlaager@wiktel.com>
[Fixed the other two instances too.]

docs/Getting Started/Debian/Debian Buster Root on ZFS.rst

@@ -650,7 +650,7 @@ Step 4: System Configuration
 
 #. Optional: Remove os-prober::
 
-     dpkg --purge os-prober
+     apt --purge os-prober
 
    This avoids error messages from `update-grub`.  `os-prober` is only
    necessary in dual-boot configurations.

docs/Getting Started/Ubuntu/Ubuntu 18.04 Root on ZFS.rst

@@ -555,7 +555,7 @@ Select (using the space bar) all of the disks (not partitions) in your pool.
 
 4.9 (Optional): Remove os-prober::
 
-    dpkg --purge os-prober
+    apt --purge os-prober
 
 This avoids error messages from `update-grub`.  `os-prober` is only necessary
 in dual-boot configurations.

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS.rst

@@ -796,7 +796,7 @@ Step 4: System Configuration
 
 #. Optional: Remove os-prober::
 
-     dpkg --purge os-prober
+     apt --purge os-prober
 
    This avoids error messages from ``update-grub``.  ``os-prober`` is only
    necessary in dual-boot configurations.