Fix autonumber man page item

[jesus2099]

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Description of your pull request and other information

In the man youtube-dl page, the autonumber description is wrong.

The output format autonumber does not start at zero (as the man page says), it starts at 00001 (as verified and as written elsewhere in the same man page).

I hope changing the README.md will change the man page. I didn't find this description anywhere else in the repo.

[dstftw]

This depends on the conversion type used.

[jesus2099]

Hello @dstftw.

I don't know in which cases it starts at zero (maybe the doc should explain that) because it always started at 00001 for me.

The issue is that these two lines, both in the same README.md, are in contradiction:

--autonumber-start NUMBER Specify the start value for %(autonumber)s (default is 1)

autonumber (numeric): Five-digit number that will be increased with each download, starting at zero

And... both are wrong if This depends on the conversion type used. 😉

[dstftw]

I mean it depends on conversion type whether it will be prefixed with leading zeros or not. It always starts at --autonumber-start.