Fix devtools documentation issue.

devToolsConnection.onDisconnect should be
devToolsConnection.onDisconnect.addListener.

BUG=526882
R=pfeldman@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1339173002

Cr-Commit-Position: refs/heads/master@{#348751}

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

@@ -209,7 +209,7 @@ chrome.runtime.onConnect.addListener(function(devToolsConnection) {
     // add the listener
     devToolsConnection.onMessage.addListener(devToolsListener);
 
-    devToolsConnection.onDisconnect(function() {
+    devToolsConnection.onDisconnect.addListener(function() {
          devToolsConnection.onMessage.removeListener(devToolsListener);
     });
 }