fix bug of disable_preview

[forestchen1224]

When open the preview,the preview is very lag,even though disable_preview is set.This is because the bug in the code that disable_preview is not used in the if statement.

    if
      entry_is_file
      and config.preview_win.preview_method ~= "load"
      and not util.file_matches_bufreadcmd(normalized_url)
      and not config.preview_win.disable_preview(normalized_url)
    then
      filebufnr =
        util.read_file_to_scratch_buffer(normalized_url, config.preview_win.preview_method)

[stevearc]

Thanks for the PR!