Update self_update.rs

Fixing janky line breaks in the self-updater path help string, as reported in rust-lang#2334

src/cli/self_update.rs

@@ -149,9 +149,9 @@ macro_rules! post_install_msg_win {
     () => {
         r"# Rust is installed now. Great!
 
-To get started you need Cargo's bin directory ({cargo_home}\\bin) in your `PATH`
-environment variable. Future applications will automatically have the
-correct environment, but you may need to restart your current shell.
+To get started you need Cargo's bin directory ({cargo_home}\\bin) in 
+your `PATH` environment variable. Future applications will automatically
+have the correct environment, but you may need to restart your current shell.
 "
     };
 }