Update let_underscore.rs

Author: saschanaz

compiler/rustc_lint/src/let_underscore.rs

@@ -11,7 +11,7 @@ declare_lint! {
     /// scope.
     ///
     /// ### Example
-    /// ```rust,compile_fail
+    /// ```
     /// struct SomeStruct;
     /// impl Drop for SomeStruct {
     ///     fn drop(&mut self) {
@@ -56,7 +56,7 @@ declare_lint! {
     /// of at end of scope, which is typically incorrect.
     ///
     /// ### Example
-    /// ```compile_fail
+    /// ```rust,compile_fail
     /// use std::sync::{Arc, Mutex};
     /// use std::thread;
     /// let data = Arc::new(Mutex::new(0));