"umask" is pronounce like "youmask", so having an "an" before it is a
bit strange. In other places in the help, "umask" is not preceded by
either "a" or "an", and sometimes preceded by "the".
Also, "Note" is usually followed either by ":" or "that" in builtin.txt,
so add a ":" after "Note".
closes: 16860
Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>