docs(layer): Fix SSM parameter name for looking up layer ARN (#6221)

Author: speshak

docs/index.md

@@ -88,7 +88,7 @@ You can install Powertools for AWS Lambda (Python) using your favorite dependenc
         Sample Placeholders:
 
         - `{arch}` is either `arm64` (Graviton based functions) or `x86_64`
-        - `{python_version}` is the Python version without the period (.), e.g., `python313` for `Python 3.13`.
+        - `{python_version}` is the Python runtime version, e.g., `python3.13` for `Python 3.13`.
         - `{version}` is the semantic version number (e,g. 3.1.0) for a release or `latest`
 
         ```yaml