[app/docs/ssl.md] Enable SSL - using python3

[43skyline]

While following the script,

sudo apt-get install certbot python-certbot-nginx

Under Ubuntu 20.04, this is returning

Package python-certbot-nginx is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
python3-certbot-nginx

It appears python-certbot-nginx is replaced with python3-certbot-ngix.

Is it all good to go with python3-certbot-ngix?

[nguyenkims]

Yes I think it's safe to go with python3-certbot-nginx