Fixing header check location for webms

[mitt3n]

Summary

Webm header check bytes location is wrong. This is untested, however:

>>> file = open("/home/user/file.webm", "rb")
>>> bytes = file.read(36)
>>> print(bytes[22:29])
b'\x82\x84webmB'

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Gobot1234]

Thanks