Subst odoc version in css file

Author: jonludlam

src/html_support_files/odoc.css

@@ -1,7 +1,7 @@
 @charset "UTF-8";
 /* Copyright (c) 2016 The odoc contributors. All rights reserved.
    Distributed under the ISC license, see terms at the end of the file.
-   %%NAME%% %%VERSION%% */
+   odoc 2.2.0 */
 
 /* Fonts */
 @import url('https://fonts.googleapis.com/css?family=Fira+Mono:400,500');