add missing 'class' keyword for consistency with other types

[hankache]

The problem

X::Method::NotFound, X::Parameter::MultipleTypeConstraints, X::Parameter::WrongOrder and X::Placeholder::Mainline have no class keyword in their =TITLE. All other classes do.

Solution provided

Add class keyword in their =TITLE

[zoffixznet]

FWIW: You should have a commit bit and can commit directly.

[hankache]

@zoffixznet I do. Just waiting for Travis to finish before I merge.

[hankache]

@zoffixznet on a side note, you're right I should commit directly without submitting a PR