prompty: docstring (#23152)

Added missed docstrings. Format docstrings to the consistent format
(used in the API Reference)

---------

Co-authored-by: ccurme <chester.curme@gmail.com>

libs/partners/prompty/langchain_prompty/renderers.py

@@ -5,6 +5,8 @@ from .core import Invoker, Prompty, SimpleModel
 
 
 class MustacheRenderer(Invoker):
+    """Render a mustache template."""
+
     def __init__(self, prompty: Prompty) -> None:
         self.prompty = prompty