Update llm_chain.ipynb text (#22198)

Added the missing verb "is" and a comma to the text in the Prompt
Templates description within the Build a Simple LLM Application tutorial
for more clarity.

docs/docs/tutorials/llm_chain.ipynb

@@ -289,7 +289,7 @@
    "source": [
     "## Prompt Templates\n",
     "\n",
-    "Right now we are passing a list of messages directly into the language model. Where does this list of messages come from? Usually it constructed from a combination of user input and application logic. This application logic usually takes the raw user input and transforms it into a list of messages ready to pass to the language model. Common transformations include adding a system message or formatting a template with the user input.\n",
+    "Right now we are passing a list of messages directly into the language model. Where does this list of messages come from? Usually, it is constructed from a combination of user input and application logic. This application logic usually takes the raw user input and transforms it into a list of messages ready to pass to the language model. Common transformations include adding a system message or formatting a template with the user input.\n",
     "\n",
     "PromptTemplates are a concept in LangChain designed to assist with this transformation. They take in raw user input and return data (a prompt) that is ready to pass into a language model. \n",
     "\n",