docs: Fixed grammar mistake (#20697)

Description: Changed "You are" to "You are a". Grammar issue.
Dependencies: None

Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

docs/docs/get_started/quickstart.mdx

@@ -194,7 +194,7 @@ Prompt templates convert raw user input to better input to the LLM.
 ```python
 from langchain_core.prompts import ChatPromptTemplate
 prompt = ChatPromptTemplate.from_messages([
-    ("system", "You are world class technical documentation writer."),
+    ("system", "You are a world class technical documentation writer."),
     ("user", "{input}")
 ])
 ```