docs: Update messages.mdx (langchain-ai#27856)

### Description
Updates phrasing for the header of the `Messages` section.

Co-authored-by: Erick Friis <erick@langchain.dev>

docs/docs/concepts/messages.mdx

@@ -12,7 +12,7 @@ Each message has a **role** (e.g., "user", "assistant"), **content** (e.g., text
 
 LangChain provides a unified message format that can be used across chat models, allowing users to work with different chat models without worrying about the specific details of the message format used by each model provider.
 
-## What inside a message?
+## What is inside a message?
 
 A message typically consists of the following pieces of information: