[Mailer] Added missing PHPDoc argument

[sweoggy]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Related tickets	N/A
License	MIT

[pamil]

Thanks @sweoggy, can you rebase it to 1.0?

[sweoggy]

@pamil done.

For anyone's reference, I did the following to rebase it:
git fetch upstream
git pull --rebase upstream 1.0
git rebase -i HEAD~3 (in case of leftovers from master branch, where 3 = the amount of bad commits + own commits)
git push --force

[pamil]

Thanks Oscar! 🎉