sql: fix flake in TestTxnContentionEventsTable

Instead of 499 ms, allow pg_sleep(.5) to sometimes take 490 ms.

Fixes: cockroachdb#118236

Release note: None

pkg/sql/crdb_internal_test.go

@@ -1019,7 +1019,7 @@ func causeContention(
 		ctx, fmt.Sprintf("UPDATE %s SET s = $1 where id = 'test';", table), updateValue)
 	require.NoError(t, errUpdate)
 	end := timeutil.Now()
-	require.GreaterOrEqual(t, end.Sub(start), 499*time.Millisecond)
+	require.GreaterOrEqual(t, end.Sub(start), 490*time.Millisecond)
 
 	wgTxnDone.Wait()
 }