Rollup merge of #155610 - Omnikar:trait-alias-docs-fix, r=mejrs

Add missing `dyn` keyword to `trait_alias` page of the Unstable Book

There seemed to be a small typo in the Rust Unstable Book page for the `trait_alias` feature, where a variable is declared as `&Bar` for a trait `Bar`, rather than `&dyn Bar`.

src/doc/unstable-book/src/language-features/trait-alias.md

@@ -26,7 +26,7 @@ pub fn main() {
     foo(&1);
 
     // Use trait alias for trait objects.
-    let a: &Bar = &123;
+    let a: &dyn Bar = &123;
     println!("{:?}", a);
     let b = Box::new(456) as Box<dyn Foo>;
     println!("{:?}", b);