[agent builder] Cleanup API reference subheading

Author: leemthompo

solutions/search/agent-builder/agent-builder-agents.md

@@ -119,6 +119,6 @@ The Agents API enables programmatic access to agent creation and management acti
 
 For an overview, refer to [Agents API](kibana-api.md#agents).
 
-### Serverless API reference
+### API reference
 
 For the complete API reference, refer to the [Kibana API reference](https://www.elastic.co/docs/api/doc/kibana/operation/operation-get-agent-builder-agents).

solutions/search/agent-builder/chat.md

@@ -66,7 +66,7 @@ The Agent Chat API provides programmatic access to chat functionality through RE
 
 For a quick overview of the REST API for conversations, refer to [Chat and conversations API](kibana-api.md#chat-and-conversations).
 
-### Serverless API reference
+### API reference
 
 For the complete API reference, refer to the [Kibana API reference](https://www.elastic.co/docs/api/doc/kibana/operation/operation-get-agent-builder-conversations).
 

solutions/search/agent-builder/tools.md

@@ -193,7 +193,7 @@ Note that built-in tools cannot be modified or deleted.
 
 For a quick overview of how to work programmatically with tools, refer to [Tools API](kibana-api.md#tools).
 
-### Serverless API reference
+### API reference
 
 For the complete API reference, refer to the [Kibana API reference](https://www.elastic.co/docs/api/doc/kibana/operation/operation-get-agent-builder-tools).