Fix events docs

[domdomegg]

As per https://github.com/axenox/onscan.js/blob/master/onscan.js#L300

[domdomegg]

@tommy-josepovic

[tjosepo]

Relevant discussion.

While adding TypeScript type definitions to this library, we only found mentions of the scanButtonLongPress event in the documentation and we did not find it in the code itself.

We believe that the scanButtonLongPress event might not exist and should be removed from the documentation to avoid confusion.