docs(header): update ContentType example to use ContentType::json()
docs(header): update ContentType example to use ContentType::json()

yazaddaruvala/hyper,hyperium/hyper,Twey/hyper,tshepang/hyper,nickgonzales/hyper,Twey/hyper,hyperium/hyper,yazaddaruvala/hyper,nickgonzales/hyper,Twey/hyper,nickgonzales/hyper,tshepang/hyper,tshepang/hyper,hyperium/hyper