GPT: fix error in partitions string doc

GPT: fix error in partitions string doc

The existing partitions-list parsing in cmd/gpt.c passes a value
from gpt_default() to set_gpt_info() that README.gpt suggests
should begin with 'partitions='.  Partition-list strings should
in fact begin with 'uuid_disk', as otherwise the call from
set_gpt_info() to extract_val() to find 'uuid_disk' will fail.
Change README.gpt accordingly.

Changes since v6: none.

Signed-off-by: Alison Chaiken <alison@peloton-tech.com>