fix doctest

Author: iSwapna

compiler/rustc_lint_defs/src/builtin.rs

@@ -718,7 +718,8 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ``` fn func() {
+    /// ``` rust
+    /// fn func() {
     ///     const CAP: usize = std::u32::MAX as usize;
     ///     let mut x: [u8; CAP>>1] = [0; CAP>>1];
     ///     x[2] = 123;