reference: fix outdated link for Compiler plugins
book/syntax-extensions.html was renamed to book/plugins.html,
the link should be also updated.
@@ -648,7 +648,7 @@ All of the above extensions are expressions with values.
Users of `rustc` can define new syntax extensions in two ways:
-* [Compiler plugins](book/syntax-extensions.html) can include arbitrary
+* [Compiler plugins][plugin] can include arbitrary
Rust code that manipulates syntax trees at compile time.
* [Macros](book/macros.html) define new syntax in a higher-level,
