docs: correct broken anchor link in py_test docstring for Bazel registry docs

The Bazel registry (registry.bazel.build) renders function/macro
definition anchors with a `function-` prefix (e.g.
`id="function-py_binary"`), but the `py_test` docstring linked to
`./py_binary.md` (a relative file path), which resolves to a 404 on
the registry.

Fix:
- `./py_binary.md` -> `#function-py_binary` in `py_test` macro
  docstring

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: georgesfarah

py/defs.bzl

@@ -143,7 +143,7 @@ def py_binary(name, srcs = [], main = None, **kwargs):
     )
 
 def py_test(name, srcs = [], main = None, pytest_main = False, **kwargs):
-    """Identical to [py_binary](./py_binary.md), but produces a target that can be used with `bazel test`.
+    """Identical to [py_binary](#function-py_binary), but produces a target that can be used with `bazel test`.
 
     Args:
         name: Name of the rule.