docs[patch]: Fix bad headers in output parser docs (#15778)
Currently looks like this:
<img width="282" alt="Screenshot 2024-01-09 at 1 08 53 PM"
src="https://github.com/langchain-ai/langchain/assets/6952323/58f3d368-6588-418e-8502-30d13757cb99">
CC @efriis @baskaryan
