Update release instructions

Spack package now at 

`git clone git@github.com:spack/spack-packages.git`

Edit: spack-packages/repos/spack_repo/builtin/packages/py_libensemble/package.py

To see libE online:
https://github.com/spack/spack-packages/blob/develop/repos/spack_repo/builtin/packages/py_libensemble/package.py

