Fix description of specs of UNIXSocket.pair

[herwinw]

This is a class method, not an instance method.

[eregon]

@herwinw Given your many great PRs & contributions I would like to give you write access to ruby/spec. And then you can merge simple PRs like this yourself or even directly push to master.

@hsbt Could you add @herwinw to the Ruby organization? Then I can add him to the rubyspec-committers team.

[hsbt]

@eregon I send an invitation to @herwinw directly joining rubyspec-committers team.

[eregon]

@herwinw Welcome to ruby/spec committers! Please merge this PR yourself :)

[herwinw]

Nailed it 😆