README.md: sample configuration toggle_telescope fix

[lalit10368]

ipairs doesn't iterate over all the keys in table. It stops when it sees the first nil (removed file). If we remove any file in the harpoon:list() we set it to nil. The harpoon's quick_menu ui shows blank for the removed file, however the toggle_telescope ui only lists file till it encounters a nil (discards any items in harpoon_files after some nil).