fix(langchain): correct createAgent parameter name in quickstart (#1157)

The createAgent function in JavaScript/TypeScript uses systemPrompt as
the parameter name, not prompt. This updates the documentation to
reflect the correct API signature.

Changes:
- Updated remaining instance in full example code section (line 535)
- Ensures consistency with the correct createAgent API

## Overview
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## Type of change

**Type:** [Replace with: New documentation page / Update existing
documentation / Fix typo/bug/link/formatting / Remove outdated content /
Other]

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## Checklist
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- [ ] I have read the [contributing guidelines](README.md)
- [ ] I have tested my changes locally using `docs dev`
- [ ] All code examples have been tested and work correctly
- [ ] I have used **root relative** paths for internal links
- [ ] I have updated navigation in `src/docs.json` if needed
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## Additional notes
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Author: nkhunters

src/oss/langchain/quickstart.mdx

@@ -337,7 +337,7 @@ Let's walk through each step:
 
         const agent = createAgent({
           model: "anthropic:claude-sonnet-4-5",
-          prompt: systemPrompt,
+          systemPrompt: systemPrompt,
           tools: [getUserLocation, getWeather],
           responseFormat,
           checkpointer,
@@ -532,7 +532,7 @@ const checkpointer = new MemorySaver();
 // Create agent
 const agent = createAgent({
   model: "anthropic:claude-sonnet-4-5",
-  prompt: systemPrompt,
+  systemPrompt: systemPrompt,
   tools: [getUserLocation, getWeather],
   responseFormat,
   checkpointer,