buildman: fix typos of --dry-run help message

try run    => dry run
no nothing => do nothing
"..."      => '...'

The last one is for consistency with the other option helps.

Change-Id: I1d69047d1fae6ef095a18f69f44ee13c448db9b7
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>