Readme: add missing dot to .mime.types

[meonkeys]

The hack only worked for me if the file was .mime.types (with a leading period).

Related: I noticed that https://github.com/asciidoctor/asciidoctor-browser-extension somehow works on plain text Asciidoc files without this or other hacks. 🤷

Also mentioned at #86 (comment)

[Cimbali]

Did you change the helpers.private_mime_types_file config value?

[meonkeys]

I did not; it was already set to ~/.mime.types so I left it as-is.

[Cimbali]

Ok so that’s why it’s looking for a .mime.types filename then :)

[meonkeys]

Ah gotcha, on my first read I missed that the Readme is suggesting changing the config value to not include the leading dot. Thank you.