docs: fix JS quickstart (googleapis#2406)

## Description

Response is being re-assigned

## PR Checklist

> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:

- [ ] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [ ] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)
- [ ] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

Author: twishabansal

docs/en/getting-started/quickstart/js/genkit/quickstart.js

@@ -53,7 +53,7 @@ export async function main() {
 
   for (const query of queries) {
     conversationHistory.push({ role: "user", content: [{ text: query }] });
-    const response = await ai.generate({
+    let response = await ai.generate({
       messages: conversationHistory,
       tools: tools,
     });