Fix "extensions" link in README.md (#4717) #9

Job	Run time
check-proto-changes-python	1m 9s
docs (./packages/autogen-core)	1m 23s
docs-example-check (./packages/autogen-core)	2m 22s
format	1m 2s
lint	54s
mypy (./packages/autogen-core)	2m 1s
pyright (./packages/autogen-core)	59s
mypy (./packages/autogen-magentic-one)	1m 10s
test (./packages/autogen-core)	1m 5s
pyright (./packages/autogen-magentic-one)	59s
mypy (./packages/agbench)	1m 3s
test (./packages/autogen-magentic-one)	1m 28s
pyright (./packages/agbench)	54s
mypy (./packages/autogen-ext)	1m 48s
test (./packages/autogen-ext)	2m 38s
pyright (./packages/autogen-ext)	1m 1s
mypy (./packages/autogen-agentchat)	1m 8s
test (./packages/autogen-agentchat)	56s
pyright (./packages/autogen-agentchat)	56s
	24m 56s

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