Fix broken link in unstable book plugin

Author: giraffate

src/doc/unstable-book/src/language-features/plugin.md

@@ -35,7 +35,7 @@ of a library.
 Plugins can extend [Rust's lint
 infrastructure](../../reference/attributes/diagnostics.md#lint-check-attributes) with
 additional checks for code style, safety, etc. Now let's write a plugin
-[`lint_plugin_test.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui-fulldeps/auxiliary/lint_plugin_test.rs)
+[`lint-plugin-test.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui-fulldeps/auxiliary/lint-plugin-test.rs)
 that warns about any item named `lintme`.
 
 ```rust,ignore