Rollup merge of #23329 - jbcrail:rm-syntax-highlight, r=sanxiyn

As suggested by @steveklabnik in #23254, I removed the redundant Rust syntax highlighting from the documentation.

src/liballoc/arc.rs

@@ -94,7 +94,7 @@ use heap::deallocate;
 /// With simple pipes, without `Arc`, a copy would have to be made for each
 /// task.
 ///
-/// ```rust
+/// ```
 /// use std::sync::Arc;
 /// use std::thread;
 ///

src/liballoc/boxed.rs

@@ -64,7 +64,7 @@ use core::raw::TraitObject;
 ///
 /// The following two examples are equivalent:
 ///
-/// ```rust
+/// ```
 /// #![feature(box_syntax)]
 /// use std::boxed::HEAP;
 ///