update nu_plugin_python due to signature changes (#8107)

# Description

As title..

# User-Facing Changes

None

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu_plugin_python/nu_plugin_python_example.py

@@ -1,3 +1,4 @@
+#!/usr/bin/env python
 # Example of using a Python script as a Nushell plugin
 #
 # The example uses JSON encoding but it should be a similar process using
@@ -38,6 +39,7 @@ def signatures():
     return {
         "Signature": [
             {
+                "sig": {
                     "name": "nu-python",
                     "usage": "Signature test for Python",
                     "extra_usage": "",
@@ -79,7 +81,7 @@ def signatures():
                             "arg": None,
                             "required": False,
                             "desc": "Display the help message for this command",
-                        "var_id": None
+                            "var_id": None,
                         },
                         {
                             "long": "flag",
@@ -105,6 +107,8 @@ def signatures():
                     "creates_scope": False,
                     "allows_unknown_args": False,
                     "category": "Experimental",
+                },
+                "examples": [],
             }
         ]
     }