Fix "extensions" link in README.md

[inbal2l]

Why are these changes needed?

"Extentions" link fix - broken link

Related issue number

Closes #4716

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.