update jemallocator example to use 2018 edition import syntax

kraktus · kraktus · commit 519aa6e4d7d6 · 2022-05-04T13:43:33.000+02:00
diff --git a/library/std/src/alloc.rs b/library/std/src/alloc.rs
@@ -42,8 +42,6 @@
 //! [`GlobalAlloc`] trait. This type can be provided by an external library:
 //!
 //! ```rust,ignore (demonstrates crates.io usage)
-//! extern crate jemallocator;
-//!
 //! use jemallocator::Jemalloc;
 //!
 //! #[global_allocator]
