fix(config): fix invalid readme reference in pyproject.toml

Not sure where the original `README-PYPI.md` file came from, maybe a bug
in initial generation?

Without these changes, we get the following error:


```
🚀  ❯ poetry install
Installing dependencies from lock file

No dependencies to install or update

Installing the current project: glean-api-client (0.5.2)
Error: The current project could not be installed: Readme path `/Users/rwjblue/src/github/gleanwork/api-client-python/README-PYPI.md` does not exist.
If you do not want to install the current project use --no-root.
If you want to use Poetry only for dependency management but not for packaging, you can disable package mode by setting package-mode = false in your pyproject.toml file.
If you did intend to install the current project, you may need to set `packages` in your pyproject.toml file.
```

Author: rwjblue-glean

pyproject.toml

@@ -4,7 +4,7 @@ version = "0.5.2"
 description = "Python Client SDK Generated by Speakeasy."
 authors = [{ name = "Glean Technologies, Inc." }]
 license = "MIT"
-readme = "README-PYPI.md"
+readme = "README.md"
 requires-python = ">=3.9"
 dependencies = ["httpx >=0.28.1", "pydantic >=2.11.2"]