Fix unmatched-organization warnings for deny template

[Marcono1234]

With the current deny.template.toml https://github.com/EmbarkStudios/cargo-deny-action prints the following warnings:

 warning[unmatched-organization]: allowed 'github.com' organization  was not encountered
    ┌─ ./deny.toml:235:11
    │
235 │ github = [""]
    │           ━ no crate source fell under this organization

warning[unmatched-organization]: allowed 'gitlab.com' organization  was not encountered
    ┌─ ./deny.toml:237:11
    │
237 │ gitlab = [""]
    │           ━ no crate source fell under this organization

warning[unmatched-organization]: allowed 'bitbucket.org' organization  was not encountered
    ┌─ ./deny.toml:239:14
    │
239 │ bitbucket = [""]
    │              ━ no crate source fell under this organization

Note: I have marked this as draft for now because I am not sure if the approach of changing the documentation to allow 0 Git sources is good here; an alternative would be to just comment out github = ["..."], ...
Though it seems 0 Git sources are currently permitted, see also #689.