Fix very small typo (#12603)

- **Description:** this is the world's smallest typo change of a typo I
saw while reading the docs

docs/docs/expression_language/cookbook/prompt_llm_parser.ipynb

@@ -30,7 +30,7 @@
    "source": [
     "## PromptTemplate + LLM\n",
     "\n",
-    "The simplest composition is just combing a prompt and model to create a chain that takes user input, adds it to a prompt, passes it to a model, and returns the raw model output.\n",
+    "The simplest composition is just combining a prompt and model to create a chain that takes user input, adds it to a prompt, passes it to a model, and returns the raw model output.\n",
     "\n",
     "Note, you can mix and match PromptTemplate/ChatPromptTemplates and LLMs/ChatModels as you like here."
    ]