fix: use correct url when using privacy mode

[12rambau]

Fix #57

I simply edit the url used when privacy_mode is activated.
@phw, could you test it against your documentation and let me know if it fixes the issue ?

[phw]

@12rambau Thanks for the patch. We are close, but not fully working :) According to the docs at https://sphinxcontrib-youtube.readthedocs.io/en/latest/usage.html you are supposed to be able to use it like this:

..  youtube:: dQw4w9WgXcQ
   :privacy_mode:

But in this case node["privacy_mode"] is an empty string. If it is not set at all it is None. Changing the line to the below I think will work as intended:

node["platform_url"] = privacy if node["privacy_mode"] is not None else embed

[phw]

The patch works for me now. Thanks for the fix.

[12rambau]

@dham ready for review