tests/rustdoc-json: directives should not use doc comments

Author: lolbinarycat

tests/rustdoc-json/fns/return_type_alias.rs

@@ -1,6 +1,6 @@
 // Regression test for <https://github.com/rust-lang/rust/issues/104851>
 
-///@ set foo = "$.index[?(@.name=='Foo')].id"
+//@ set foo = "$.index[?(@.name=='Foo')].id"
 pub type Foo = i32;
 
 //@ is "$.index[?(@.name=='demo')].inner.function.sig.output.resolved_path.id" $foo