Edits dom/bindings/DESIGN.md

This corrects and improves the dom/bindings/DESIGN.md document and also
corrects a grammatical error in a tests/reftest.rs message.
auto merge of #3720 : gilles-leblanc/servo/fix-grammatical-errors, r=jdm

This corrects and improves the dom/bindings/DESIGN.md document and also
corrects a grammatical error in a tests/reftest.rs message.