Fix some typos, broken links and other issues in extension docs

chrome/common/extensions/docs/templates/articles/packaging.html

 <h1>Packaging</h1>
 
 
 <p>
 This page describes how to package your extension.
 As the <a href="overview.html">Overview</a> explains,
 extensions are packaged as signed ZIP files
 with the file extension "crx"&mdash;for example,
 <code>myextension.crx</code>.
 </p>
@@ skipping 28 matching lines @@
 <ol>
   <li>
     Bring up the Extensions management page
     by going to this URL:
     <blockquote>
     <b>chrome://extensions</b>
     </blockquote>
   </li>
 
   <li>
-    If <b>Developer mode</b> has a + by it,
-    click the +.
+    Ensure that the "Developer mode" checkbox in the top right-hand corner
+    is checked.
   </li>
 
   <li>
     Click the <b>Pack extension</b> button.
     A dialog appears.
   </li>
 
   <li>
     In the <b>Extension root directory</b> field,
     specify the path to the extension's folder&mdash;for example,
@@ skipping 120 matching lines @@
 
 <pre>
 chrome.exe --pack-extension=c:\myext --pack-extension-key=c:\myext.pem
 </pre>
 
 <h2 id="format">Package format and scripts</h2>
 <p>
 For more information on the format, as well as pointers to scripts you can use
 to create <code>.crx</code> files, see <a href="crx.html">CRX Package Format</a>.
 </p>