The path URL is not real URL address, it's dummy URL address, which made for marketing purpose, so user can understand your ads in better way and you get more clicks.
For example this URL is very buggy for ads
What if you can display below URL in your ad campaign, behind your actual URL.
It's look great then above URL. And you will get more clicks.
So the path is not real and that's why they might not allowed to use dot .(Sorry I have never tried like that) . The actual path or your landing page URL (ln this example www.example.com/contact/) should be given in final URL textbox, so whenever anybody click on your ads then that final URL will be open on their browser.
Here is more details about paths in adwords ads.