PHPMailer: Remove duplicate array keys

Summary:
'doc' is listed twice in the $mimes array. Remove one key.

The second key was also removed in https://github.com/PHPMailer/PHPMailer/commit/6f919df65e80023fce4eab58832b6035b4a41935.

Test Plan: Read/grep the code.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25666