Fix document for crate_universe (#3550)

It's `use_repo(crate, "crates")` for `bzlmod`
There's no `crate_index`

The example code is also using `crates`

https://github.com/bazelbuild/rules_rust/blob/a44ac6e1bc66edcd4e45b9277eb51ff0981de742/examples/all_crate_deps/BUILD.bazel#L2

Author: powei-lin

crate_universe/extensions.bzl

@@ -81,7 +81,7 @@ they return will automatically update BUILD targets. In your BUILD files,
 you use these macros for a Rust library as shown below:
 
 ```python
-load("@crate_index//:defs.bzl", "aliases", "all_crate_deps")
+load("@crates//:defs.bzl", "aliases", "all_crate_deps")
 load("@rules_rust//rust:defs.bzl", "rust_library", "rust_test")
 
 rust_library(