R1:eecf6b8ecabe

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out of date, and to see GitHub instead.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

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