Fix the old clippy invocation

dtolnay · dtolnay · commit 047d4085ef64 · 2018-12-06T09:50:23.000-08:00
diff --git a/posts/2018-12-06-Rust-1.31-and-rust-2018.md b/posts/2018-12-06-Rust-1.31-and-rust-2018.md
@@ -361,7 +361,7 @@ If you previously used Clippy's lints, you can migrate like this:
 
 ```rust
 // old
-#![cfg_attr(clippy, bool_comparison)]
+#![cfg_attr(feature = "cargo-clippy", allow(bool_comparison))]
 
 // new
 #![allow(clippy::bool_comparison)]
