fix double $ in the 1.39.0 blog post

Author: pietroalbini

posts/2019-11-07-Rust-1.39.0.md

@@ -10,7 +10,7 @@ The Rust team is happy to announce a new version of Rust, 1.39.0. Rust is a prog
 If you have a previous version of Rust installed via rustup, getting Rust 1.39.0 is as easy as:
 
 ```console
-$ rustup update stable
+rustup update stable
 ```
 
 If you don't have it already, you can [get `rustup`][install] from the appropriate page on our website, and check out the [detailed release notes for 1.39.0][notes] on GitHub.