Fix config annotation for note_frontmatter_func

[parmort]

Running the setup function as follows would throw an LSP warning because no return type was given in the annotation for note_frontmatter_func. I just added the return type of table to the field annotation.

require('obsidian').setup({
  note_frontmatter_func = function(note)
    return {}
  end
})

[epwalsh]

Thanks!