fix a small typo in an error message.

collector/diskstats_linux.go

@@ -258,7 +258,7 @@ func parseDiskStats(r io.Reader) (map[string]map[int]string, error) {
 
 		bytesWritten, err := convertDiskSectorsToBytes(diskStats[dev][6])
 		if err != nil {
-			return nil, fmt.Errorf("invalid value for sectors read in %s: %s", procFilePath("diskstats"), scanner.Text())
+			return nil, fmt.Errorf("invalid value for sectors written in %s: %s", procFilePath("diskstats"), scanner.Text())
 		}
 		diskStats[dev][12] = bytesWritten
 	}