As Bill said this branch doesn’t exist in the polkadot-sdk monorepo.
I have seen this issue before on StackExchange: What version of polkadot relaychain and substrate-parachain-template can be used? and what I did was to open a fix in the Substrate Docs repository: Fix wrong branch of new repository fixing that.
I would suggest you open an issue or create a fix yourself to help the Documentation Team: Sign in to GitHub · GitHub
For the future, this kind of issues are better to ask in StackExchange than here in the Forum.