Fix llama2 README.md cmake instructions (#3096)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3096

As titled. The current instruction runs into issue due to our way of arranging `pthreadpool` and `cpuinfo` in CMake. Will need a bigger effort to clean them up. For now let's update the instruction to be able to run it.

Reviewed By: mergennachin

Differential Revision: D56251563

fbshipit-source-id: daf0b1ecb75abb90612efbd64108edc99a129efd