Remove package-workspace from unstable doc index

This was stabilized in 1.90, but the index entry wasn't removed.

Author: ehuss

src/doc/src/reference/unstable.md

@@ -125,7 +125,6 @@ Each new feature described below should explain how to use it.
     * [gitoxide](#gitoxide) --- Use `gitoxide` instead of `git2` for a set of operations.
     * [script](#script) --- Enable support for single-file `.rs` packages.
     * [lockfile-path](#lockfile-path) --- Allows to specify a path to lockfile other than the default path `<workspace_root>/Cargo.lock`.
-    * [package-workspace](#package-workspace) --- Allows for packaging and publishing multiple crates in a workspace.
     * [native-completions](#native-completions) --- Move cargo shell completions to native completions.
     * [warnings](#warnings) --- controls warning behavior; options for allowing or denying warnings.
     * [Package message format](#package-message-format) --- Message format for `cargo package`.