build: Correct explanation of unnest-vars example

Cc: Fam Zheng <fam@euphon.net>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190213130240.15492-1-armbru@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>