Update name of active.rs to unstable.rs

This file was renamed in rust-lang/rust#116550

Author: nicholasbishop

Diff for: src/feature-gates.md

@@ -20,7 +20,7 @@ See ["Stability in code"][adding] in the "Implementing new features" section for
 
 To remove a feature gate, follow these steps:
 
-1. Remove the feature gate declaration in `rustc_feature/src/active.rs`.
+1. Remove the feature gate declaration in `rustc_feature/src/unstable.rs`.
    It will look like this:
 
    ```rust,ignore
@@ -45,7 +45,7 @@ To remove a feature gate, follow these steps:
 To rename a feature gate, follow these steps (the first two are the same steps
 to follow when [removing a feature gate][removing]):
 
-1. Remove the old feature gate declaration in `rustc_feature/src/active.rs`.
+1. Remove the old feature gate declaration in `rustc_feature/src/unstable.rs`.
    It will look like this:
 
    ```rust,ignore
@@ -64,7 +64,7 @@ to follow when [removing a feature gate][removing]):
    ```
 
 3. Add a feature gate declaration with the new name to
-   `rustc_feature/src/active.rs`. It should look very similar to the old
+   `rustc_feature/src/unstable.rs`. It should look very similar to the old
    declaration:
 
    ```rust,ignore

Diff for: src/implementing_new_features.md

@@ -123,7 +123,7 @@ a new unstable feature:
 
 1. Add the feature name to `rustc_span/src/symbol.rs` in the `Symbols {...}` block.
 
-1. Add a feature gate declaration to `rustc_feature/src/active.rs` in the active
+1. Add a feature gate declaration to `rustc_feature/src/unstable.rs` in the active
    `declare_features` block.
 
    ```rust ignore