Fix bug when fetching entries

This bug caused certspotter to always request 1000 entries even if
went beyond the size of the log.  This would have prevented
certspotter from downloading entries near the end of the log, if the log was
strict with get-entries bounds.

In practice, none of the active CT logs are strict with get-entries bounds,
and even if a log were strict, certspotter would have been able to successfully
download the entries later once the log grew.

monitor/monitor.go

@@ -234,7 +234,7 @@ func monitorLog(ctx context.Context, config *Config, ctlog *loglist.Log, logClie
 
 func downloadEntries(ctx context.Context, logClient *client.LogClient, entriesChan chan<- client.GetEntriesItem, begin, end uint64) error {
 	for begin < end && ctx.Err() == nil {
-		size := begin - end
+		size := end - begin
 		if size > maxGetEntriesSize {
 			size = maxGetEntriesSize
 		}