handbook/boot: fix unload + load example

The unload + load example in the 12.2.3 section doesn't seem to work.
The original problem was reported in https://reviews.freebsd.org/D24960.

Correct the example by using fully qualified paths since this is how it works
now. Tested in 12.1, 12.2 and 13-current.

Approved by: docs (gbe@)
Differential Revision: https://reviews.freebsd.org/27277