fix: fix the system message in simple-chatbot example #2784

Job	Run time
checks / pre-commit	29s
checks / readme-snippets	14s
checks / test (3.10, lowest-direct, ubuntu-latest)	37s
checks / test (3.10, lowest-direct, windows-latest)	1m 16s
checks / test (3.11, lowest-direct, windows-latest)	1m 16s
checks / test (3.11, highest, ubuntu-latest)	35s
checks / test (3.11, highest, windows-latest)	1m 22s
checks / test (3.11, lowest-direct, ubuntu-latest)	36s
checks / test (3.12, lowest-direct, windows-latest)	1m 24s
checks / test (3.12, highest, ubuntu-latest)	33s
checks / test (3.10, highest, ubuntu-latest)	33s
checks / test (3.12, highest, windows-latest)	1m 28s
checks / test (3.12, lowest-direct, ubuntu-latest)	32s
checks / test (3.13, lowest-direct, ubuntu-latest)	36s
checks / test (3.13, lowest-direct, windows-latest)	1m 29s
checks / test (3.13, highest, ubuntu-latest)	34s
checks / test (3.10, highest, windows-latest)	1m 41s
checks / test (3.13, highest, windows-latest)	1m 55s
	17m 10s

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