Fix typo in SYSTEM_PROMPT in chatsh.mjs

I think it should be 'example.txt' here:
```
<USER>
Show me local files.
</USER>

<ChatSH>
\`\`\`sh
ls
\`\`\`
</ChatSH>

<SYSTEM>
example.gif ---- example.tst ---- example_dir/
</SYSTEM>

<USER>
Is there any text file?
</USER>

<ChatSH>
Yes, there is a file named example.txt in the current directory
</ChatSH>
```