Update README with clearer studio setup instructions

[mcavdar]

If you don’t run cd first, the command executes in lca-langgraph-essentials/python instead of lca-langgraph-essentials/python/studio, which results in the following error:

Usage: langgraph dev [OPTIONS]
Try 'langgraph dev --help' for help.

Error: Invalid value for '--config': Path 'langgraph.json' does not exist.

By the way, the repository is also missing the studio folder. Should we create one similar to this directory