last: (man) correct the descriptions of --present and --since

last: (man) correct the descriptions of --present and --since

Doing `last --present -24h` will show who was logged in around this time
yesterday.  But when doing (what according to the man page is equivalent)
`last --since -24h --until -24h` nothing will be shown -- unless someone
logged in exactly 24 hours ago to the second.

So, correct the descriptions of --present and --since.

This fixes #1896 (https://github.com/util-linux/util-linux/issues/1896).
Reported-by: Finn Krein-Schuch
Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>