Fix "extensions" link in README.md #2472

Job	Run time
format	59s
lint	1m 2s
mypy (./packages/autogen-core)	2m 0s
mypy (./packages/autogen-magentic-one)	1m 6s
mypy (./packages/agbench)	52s
mypy (./packages/autogen-ext)	1m 36s
mypy (./packages/autogen-agentchat)	1m 1s
check-proto-changes-python	1m 6s
docs (./packages/autogen-core)	1m 20s
docs-example-check (./packages/autogen-core)	2m 9s
pyright (./packages/autogen-core)	57s
pyright (./packages/autogen-magentic-one)	57s
pyright (./packages/agbench)	53s
pyright (./packages/autogen-ext)	59s
pyright (./packages/autogen-agentchat)	1m 2s
test (./packages/autogen-core)	48s
test (./packages/autogen-magentic-one)	1m 25s
test (./packages/autogen-ext)	2m 44s
test (./packages/autogen-agentchat)	1m 4s
	24m 0s

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