Fix problem with header id generation

Header id generation should match the grow logic.
All characters but whitespace are allowed
See ampproject/amp.dev#1892
And grow/grow@62d0a6a#diff-506b8f06fee5ce1722a3c180ab3919b7

lib/CodeSection.js

@@ -20,7 +20,7 @@ const hljs = require('highlight.js');
 const marked = require('marked');
 const renderer = new marked.Renderer();
 renderer.heading = function (text, level) {
-  const escapedText = text.toLowerCase().replace(/[^\w]+/g, '-');
+  const escapedText = text.toLowerCase().replace(/\s+/g, '-');
   return '<h' + level + ' id="' + escapedText +
     '" class="www-heading pb4 mb2 relative h3">' + text + '</h' + level +
     '>';
@@ -245,7 +245,7 @@ module.exports = class CodeSection {
     }
     const name = matches[1].trim().replace(/`/g, '');
     const heading = {
-      id: name.toLowerCase().replace(/[^\w]+/g, '-'),
+      id: name.toLowerCase().replace(/\s+/g, '-'),
       name: name
     };
     this.headings.push(heading);