Fix spelling mistake

src/consts.rs

@@ -484,7 +484,7 @@ fn fetch_path(&mut self, e: &Expr) -> Option<Constant> {
             if let Some(&PathResolution { base_def: DefConst(id), ..}) = lcx.tcx.def_map.borrow().get(&e.id) {
                 maybe_id = Some(id);
             }
-            // separate if lets to avoid doubleborrowing the defmap
+            // separate if lets to avoid double borrowing the def_map
             if let Some(id) = maybe_id {
                 if let Some(const_expr) = lookup_const_by_id(lcx.tcx, id, None) {
                     let ret = self.expr(const_expr);