improve docstring of doc formatting (#7162)

so it shows up nice

langchain/schema/prompt_template.py

@@ -164,6 +164,7 @@ def format_document(doc: Document, prompt: BasePromptTemplate) -> str:
 
     Example:
         .. code-block:: python
+
             from langchain.schema import Document
             from langchain.prompts import PromptTemplate
             doc = Document(page_content="This is a joke", metadata={"page": "1"})