docs: Correct WebBaseLoader URL: docs: python.langchain.com/docs/get_started/quickstartQuickstart (#17981)

**Description:** 
The URL of the data to index, specified to `WebBaseLoader` to import is
incorrect, causing the `langsmith_search` retriever to return a `404:
NOT_FOUND`.
Incorrect URL: https://docs.smith.langchain.com/overview
Correct URL: https://docs.smith.langchain.com

**Issue:** 
This commit corrects the URL and prevents the LangServe Playground from
returning an error from its inability to use the retriever when
inquiring, "how can langsmith help with testing?".

**Dependencies:** 
None.

**Twitter Handle:** 
@ryanmeinzer

docs/docs/get_started/quickstart.mdx

@@ -224,7 +224,7 @@ After that, we can import and use WebBaseLoader.
 
 ```python
 from langchain_community.document_loaders import WebBaseLoader
-loader = WebBaseLoader("https://docs.smith.langchain.com")
+loader = WebBaseLoader("https://docs.smith.langchain.com/user_guide")
 
 docs = loader.load()
 ```
@@ -543,7 +543,7 @@ from langchain_core.messages import BaseMessage
 from langserve import add_routes
 
 # 1. Load Retriever
-loader = WebBaseLoader("https://docs.smith.langchain.com/overview")
+loader = WebBaseLoader("https://docs.smith.langchain.com/user_guide")
 docs = loader.load()
 text_splitter = RecursiveCharacterTextSplitter()
 documents = text_splitter.split_documents(docs)