Fix ItemTreeManager registerColumns example

[Dominic-DallOsto]

The width parameter should be a string, but it was a number

[Dominic-DallOsto]

Is there a reason that width is a string and minWidth a number?

[windingwind]

I looked into the source code of Zotero and it seems like the type def is from https://github.com/zotero/zotero/blob/42219772a73a0d7270d52494992aad6989f4c418/chrome/content/zotero/itemTreeColumns.jsx#L351-L352

But actually I guess it does not matter, as the width/minWidth will be wrapped with parseInt and both number/string would work.

[windingwind]

Thanks for the finding. We can mark them all as number.

[Dominic-DallOsto]

Sorry for the delay - is that better?

[windingwind]

Thanks for the PR:)