update link to command reference (#8111)

# Description

Broken link

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

README.md

@@ -33,7 +33,7 @@ This project has reached a minimum-viable-product level of quality. Many people
 
 ## Learning About Nu
 
-The [Nushell book](https://www.nushell.sh/book/) is the primary source of Nushell documentation. You can find [a full list of Nu commands in the book](https://www.nushell.sh/book/command_reference.html), and we have many examples of using Nu in our [cookbook](https://www.nushell.sh/cookbook/).
+The [Nushell book](https://www.nushell.sh/book/) is the primary source of Nushell documentation. You can find [a full list of Nu commands in the book](https://www.nushell.sh/commands/), and we have many examples of using Nu in our [cookbook](https://www.nushell.sh/cookbook/).
 
 We're also active on [Discord](https://discord.gg/NtAbbGn) and [Twitter](https://twitter.com/nu_shell); come and chat with us!