Add "px" unit to image sizes in readme (#5540)

Summary:
Adding "px" unit for PyTorch site (i.e. https://pytorch.org/executorch/main/llm/llama-demo-android.html) will have same image widths as readme in github

Pull Request resolved: https://github.com/pytorch/executorch/pull/5540

Reviewed By: Riandy, kirklandsign

Differential Revision: D63226892

Pulled By: cmodi-meta

fbshipit-source-id: 5cfa30ee0ab156c1e004405cdc7dd99a0f61d2c2