Langchain: Fix quickstart doc code not working (#15352)

The quickstart doc is missing a few but very simple things that without
them, the code does not work. This PR fixes that by
- Adding commands to install `tiktoken` and `langchainhub`
- Adds a comma between 2 parameters for one of the methods

docs/docs/get_started/quickstart.mdx

@@ -223,6 +223,7 @@ First we need to install the required packages for that:
 
 ```shell
 pip install docarray
+pip install tiktoken
 ```
 
 Then we can build our index:
@@ -313,7 +314,7 @@ from langchain_core.prompts import MessagesPlaceholder
 
 prompt = ChatPromptTemplate.from_messages([
     MessagesPlaceholder(variable_name="chat_history"),
-    ("user", "{input}")
+    ("user", "{input}"),
     ("user", "Given the above conversation, generate a search query to look up in order to get information relevant to the conversation")
 ])
 retriever_chain = create_history_aware_retriever(llm, retriever, prompt)
@@ -403,6 +404,13 @@ tools = [retriever_tool, search]
 
 Now that we have the tools, we can create an agent to use them. We will go over this pretty quickly - for a deeper dive into what exactly is going on, check out the [Agent's Getting Started documentation](/docs/modules/agents)
 
+Install langchain hub first
+```bash
+pip install langchainhub
+```
+
+Now we can use it to get a predefined prompt
+
 ```python
 from langchain.chat_models import ChatOpenAI
 from langchain import hub