docs: Add missing uv command for docs generation in contributing guide

[terrytangyuan]

What does this PR do?

$ ./docs/openapi_generator/run_openapi_generator.sh
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/yutang/repos/llama-stack/docs/openapi_generator/generate.py", line 16, in <module>
    import fire
ModuleNotFoundError: No module named 'fire'

Test Plan

Tested the command uv run ./docs/openapi_generator/run_openapi_generator.sh successfully.

[ashwinb]

I had no clue you could run arbitrary scripts via uv run

[terrytangyuan]

uv is a gem!