Fix: Properly await `agent.run()` in README `Hello World` example #3358

Job	Run time
format	1m 32s
lint	58s
pyright (./packages/autogen-core)	1m 5s
pyright (./packages/autogen-magentic-one)	1m 6s
pyright (./packages/agbench)	1m 1s
check-proto-changes-python	1m 14s
pyright (./packages/autogen-ext)	1m 8s
pyright (./packages/autogen-agentchat)	1m 2s
pyright (./packages/magentic-one-cli)	1m 7s
codecov (./packages/autogen-core)	1m 15s
docs (./packages/autogen-core)	1m 24s
codecov (./packages/autogen-magentic-one)	1m 47s
docs-example-check (./packages/autogen-core)	2m 35s
codecov (./packages/autogen-ext)	2m 37s
markdown-code-lint	2m 4s
codecov (./packages/autogen-agentchat)	1m 2s
mypy (./packages/autogen-core)	2m 7s
samples-code-check	1m 0s
mypy (./packages/autogen-magentic-one)	1m 14s
test (./packages/autogen-core)	55s
mypy (./packages/agbench)	55s
test (./packages/autogen-magentic-one)	1m 19s
mypy (./packages/autogen-ext)	2m 1s
test (./packages/autogen-ext)	3m 2s
mypy (./packages/autogen-agentchat)	1m 6s
test (./packages/autogen-agentchat)	1m 8s
mypy (./packages/magentic-one-cli)	1m 4s
	38m 48s

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