docs: fix variable name typo in example code (#20658)

This pull request corrects a mistake in the variable name within the
example code. The variable doc_schema has been changed to dog_schema to
fix the error.

libs/langchain/langchain/chains/structured_output/base.py

@@ -268,7 +268,7 @@ def create_structured_output_runnable(
 
                 llm = ChatOpenAI(model="gpt-3.5-turbo-0125", temperature=0)
                 structured_llm = create_structured_output_runnable(
-                    doc_schema, 
+                    dog_schema, 
                     llm, 
                     mode="openai-tools", 
                     enforce_function_usage=True,