fix: add additional instructions to encourage output formatting

src/ragas/prompt/pydantic_prompt.py

@@ -82,6 +82,7 @@ def to_string(self, data: t.Optional[InputModel] = None) -> str:
                 if data is not None
                 else "input: (None)\n"
             )
+            + "Respond only with a valid JSON object that complies with the specified schema.\n"
             + "output: "
         )