Ubuntu: bpool must be named bpool

Fixes #85
Reported-by: Bob Ziuchkovski <bob.ziuchkovski@gmail.com>

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

@@ -432,8 +432,9 @@ Step 2: Disk Formatting
    - For raidz topologies, replace ``mirror`` in the above command with
      ``raidz``, ``raidz2``, or  ``raidz3`` and list the partitions from
      additional disks.
-   - The pool name is arbitrary. If changed, the new name must be used
-     consistently. The ``bpool`` convention originated in this HOWTO.
+   - The boot pool name is no longer arbitrary.  It _must_ be ``bpool``.
+     If you really want to rename it, edit ``/etc/grub.d/10_linux_zfs`` later,
+     after GRUB is installed (and run ``update-grub``).
 
    **Feature Notes:**