fix: Update sed to remove user as well

[OZoneGuy]

gist only wants the ID without the owner/user

Replaced sed delimiters for better readability

See #17 for context.

Tested by running gist manually with the values created in the /etc/pug file vs just the ID.

Thanks @Raptorox!

[AmmoniumX]

FYI, this wasn't working for me, it ended up writing the full URL to the file:

GIST_NAT=https://gist.github.com/user/token
GIST_AUR=https://gist.github.com/user/token

I had to slightly modify the sed command to make it work:

sed 's_https://gist.github.com/[^/]*/__g' > "${pkgdir}/etc/pug";
...
sed 's_https://gist.github.com/[^/]*/__g' >> "${pkgdir}/etc/pug";

It seems that sed didn't like the .*? part of the original command, I don't think that's valid syntax in standard sed.