t0101: use absolute date

The original version used relative approxidates, which don't
reproduce as reliably as absolute ones. Commit 6c647a fixed
this for one case, but missed the "silly" case.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t0101-at-syntax.sh

@@ -31,7 +31,7 @@ test_expect_success '@{2001-09-17} (before the first commit) shows old' '
 '
 
 test_expect_success 'silly approxidates work' '
-	check_at @{3.hot.dogs.and.30.years.ago} one
+	check_at @{3.hot.dogs.on.2001-09-17} one
 '
 
 test_expect_success 'notice misspelled upstream' '