[README.md] Fix autonumber sequence description (refs #26686)

README.md

@@ -545,7 +545,7 @@ The basic usage is not to set any template arguments when downloading a single f
  - `extractor` (string): Name of the extractor
  - `extractor_key` (string): Key name of the extractor
  - `epoch` (numeric): Unix epoch when creating the file
- - `autonumber` (numeric): Five-digit number that will be increased with each download, starting at zero
+ - `autonumber` (numeric): Number that will be increased with each download, starting at `--autonumber-start`
  - `playlist` (string): Name or id of the playlist that contains the video
  - `playlist_index` (numeric): Index of the video in the playlist padded with leading zeros according to the total length of the playlist
  - `playlist_id` (string): Playlist identifier