Fix syntax highlighting of code fence

The documentation for RESULT_EXPECT_USED includes this code:

    let res: Result<usize, ()> = Ok(1);
    res?;
    # Ok::<(), ()>(())

Because the code fence didn't start with `rust`, the code wasn't highlighted and the line starting with `#` was displayed on the website. This is now fixed.

clippy_lints/src/methods/mod.rs

@@ -143,7 +143,7 @@ declare_clippy_lint! {
     ///
     /// Better:
     ///
-    /// ```
+    /// ```rust
     /// let res: Result<usize, ()> = Ok(1);
     /// res?;
     /// # Ok::<(), ()>(())