refactor(googlereader): replace `Sprintf("%d")` with `FormatInt`

see https://medium.com/swlh/bad-go-frivolous-sprintf-2ad28fedf1a0

internal/googlereader/handler.go

@@ -769,8 +769,8 @@ func (h *handler) streamItemContentsHandler(w http.ResponseWriter, r *http.Reque
 			ID:            convertEntryIDToLongFormItemID(entry.ID),
 			Title:         entry.Title,
 			Author:        entry.Author,
-			TimestampUsec: fmt.Sprintf("%d", entry.Date.UnixMicro()),
-			CrawlTimeMsec: fmt.Sprintf("%d", entry.CreatedAt.UnixMilli()),
+			TimestampUsec: strconv.FormatInt(entry.Date.UnixMicro(), 10),
+			CrawlTimeMsec: strconv.FormatInt(entry.CreatedAt.UnixMilli(), 10),
 			Published:     entry.Date.Unix(),
 			Updated:       entry.ChangedAt.Unix(),
 			Categories:    categories,