docs: fix vim error 21 when opening help file

Attempting to open the documentation with `:help buftabline` in vim
results in the following error being displayed:

    Error detected while processing modelines:
    line 123:
    E21: Cannot make changes, 'modifiable' is off: fenc=utf-8

Removing 'fenc=utf-8' from the help file fixes the error as help files
are by default unmodifiable.

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Do you get this when you try to open the documentation for this plugin? Because otherwise it's probably an issue with my vim installation and this pr should be ignored.