doc: fix heading level (opea-project#759)

Signed-off-by: David B. Kinder <[email protected]>
Co-authored-by: David B. Kinder <[email protected]>

Author: dbkinder

comps/agent/langchain/README.md

@@ -44,7 +44,7 @@ Currently we have implemented OpenAI chat completion compatible API for agents.
 
 ## 🚀2. Start Agent Microservice
 
-#### 2.1 Build Microservices
+### 2.1 Build Microservices
 
 ```bash
 cd GenAIComps/ # back to GenAIComps/ folder