docs: Added `langchainhub` install and fixed typo (#17985)

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

docs/docs/expression_language/cookbook/agent.ipynb

@@ -7,7 +7,7 @@
    "source": [
     "# Agents\n",
     "\n",
-    "You can pass a Runnable into an agent."
+    "You can pass a Runnable into an agent. Make sure you have `langchainhub` installed: `pip install langchainhub`"
    ]
   },
   {
@@ -98,7 +98,7 @@
    "source": [
     "Building an agent from a runnable usually involves a few things:\n",
     "\n",
-    "1. Data processing for the intermediate steps. These need to represented in a way that the language model can recognize them. This should be pretty tightly coupled to the instructions in the prompt\n",
+    "1. Data processing for the intermediate steps. These need to be represented in a way that the language model can recognize them. This should be pretty tightly coupled to the instructions in the prompt\n",
     "\n",
     "2. The prompt itself\n",
     "\n",