Update git repo URLs in readme

Fixes this error:

```
git submodule add https://github.com/onweru/compose/ themes/compose
Cloning into '/home/g/src/help/themes/compose'...
ERROR: Repository not found.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: clone of 'https://github.com/onweru/compose/' into submodule path '/home/g/src/help/themes/compose' failed
```