nit: use Self in Cfg::simplify_with

Author: poliorcetics

src/librustdoc/clean/cfg.rs

@@ -224,7 +224,7 @@ impl Cfg {
     /// `Cfg`.
     ///
     /// See `tests::test_simplify_with` for examples.
-    pub(crate) fn simplify_with(&self, assume: &Cfg) -> Option<Cfg> {
+    pub(crate) fn simplify_with(&self, assume: &Self) -> Option<Self> {
         if self == assume {
             return None;
         }