Fix broken link to PHPExcel site

Summary:
phpexcel.net currently serves a 500 page, and the top Google hit for
PHPExcel (on codeplex) gives you a site warning you that it is 3 years
out of date, and to see GitHub instead.

Update the link from Maniphest's 'please install PHPExcel to enable
export' prompt.

Test Plan:
- Click 'Export to Excel' from Maniphest query result page without PHPExcel installed
- Follow suggested link to install it
- See code rather than a HTTP 500 error

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D16363

src/applications/maniphest/controller/ManiphestExportController.php

@@ -31,7 +31,9 @@ final class ManiphestExportController extends ManiphestController {
         '<p>%s</p>'.
         '<br />'.
         '<p>'.
-          '<a href="http://www.phpexcel.net/">http://www.phpexcel.net/</a>'.
+          '<a href="https://github.com/PHPOffice/PHPExcel">'.
+            'https://github.com/PHPOffice/PHPExcel'.
+          '</a>'.
         '</p>'.
         '<br />'.
         '<p>%s</p>',