Non-existing repositories may cause errors

[jingfelix]

In https://pdm-project.org/latest/usage/template/, The documentation recommends using pdm init flask to initialize a repository using a template, but in fact this repository does not exist (or is not publicly available)

I'm not sure what's happened to this repository, but I think we can change out the documentation here, like switching to django templates as examples.

[jingfelix]

I notice that the template url has been modified in this commit, while pdm init flask still remains unchanged, which should be pdm init django 👀