fix: update pybind11 git URL to the working one

[flyingmutant]

$ git clone https://github.com/pybind/pybind11/
Cloning into 'pybind11'...
ERROR: Repository not found.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

[maltemoeser]

Hmm that shouldn't be necessary, it works fine for me:

➜ git clone https://github.com/pybind/pybind11/
Cloning into 'pybind11'...
remote: Enumerating objects: 11253, done.
remote: Total 11253 (delta 0), reused 0 (delta 0), pack-reused 11253
Receiving objects: 100% (11253/11253), 4.14 MiB | 10.30 MiB/s, done.
Resolving deltas: 100% (7598/7598), done.