gpart(8): slices are out of context for GPT

Correct the GPT example. Creation of a partition leaves free the rest
of the device (not the slice).

Approved-by: imp
Fixes: ae1b731b5df0 Rewrite the GPT and MBR examples. For GPT, ensure that the boot partition is large enough for gptzfsboot, which has doubled in size since 10.
MFC after: 1 week
Pull-request: https://github.com/freebsd/freebsd-src/pull/795

(cherry picked from commit 3541d90836c0dde9734ea776f2b2b6c4ed8fd7f4)