Use doc, not test for fn item inner attr example

Using `test` causes a deny-by-default lint to trigger. See
rust-lang/rust#79003 for more information.

src/items/functions.md

@@ -295,7 +295,7 @@ available while running tests.
 
 ```rust
 fn test_only() {
-    #![test]
+    #![doc("Example")]
 }
 ```