Workaround pulldown-cmark/pulldown-cmark#124 which has broken rendering of #1990.

Author: kennytm

text/1990-external-doc-attribute.md

@@ -1,12 +1,12 @@
 <!---
-Copyright 2017 The Rust Project Developers. See the COPYRIGHT
-file at the top-level directory of this distribution.
-
-Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-<LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-option. This file may not be copied, modified, or distributed
-except according to those terms.
+-- Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+-- file at the top-level directory of this distribution.
+--
+-- Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+-- http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+-- <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+-- option. This file may not be copied, modified, or distributed
+-- except according to those terms.
 -->
 
 - Feature Name: external_doc