Fix php.net link to open in a new window

Summary:
From @chad. This setup link should open in a new window so you don't lose your context in resolving setup issues.

@chad, this was the only one I could find immediately, let me know if you remember seeing others that I missed.

Test Plan: Faked an error, clicked the link, got a new tab.

Reviewers: chad, btrahan

Reviewed By: chad

CC: chad, aran

Differential Revision: https://secure.phabricator.com/D7991