examples/ollama_functions_example: improve system message for better response

[shmsr]

For llama3, the example seemed to work but for some other models like qwen2 and friends, it was not working properly. I faced issues where getCurrentWeather tool worked but not the finalResponse tool. With these changes, I can get the desired behavior.

PR Checklist

• [x] Read the Contributing documentation.
• [x] Read the Code of conduct documentation.
• [x] Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• [ ] Describes the source of new concepts.
• [ ] References existing implementations as appropriate.
• [ ] Contains test coverage for new functions.
• [x] Passes all golangci-lint checks.

[shmsr]

@corani Can you PTAL?

[shmsr]

friendly ping

[corani]

Looks fine to me, but it's up to @tmc to decide 😉