Update PULL_REQUEST_TEMPLATE.md
Update PULL_REQUEST_TEMPLATE.md

Fix typos and make the checklist into an actual checklist
reactjs/react-modal,dminuoso/react-modal,bluSCALE4/react-modal,mgh/react-modal,diasbruno/react-modal,seanich/react-modal,d-mon-/react-modal,oisu/react-modal,bluSCALE4/react-modal,diasbruno/react-modal,reactjs/react-modal,reactjs/react-modal,seanich/react-modal,ineo6/react-modal,springload/react-modal,diasbruno/react-modal,seanich/react-modal,oisu/react-modal,bluSCALE4/react-modal,d-mon-/react-modal,springload/react-modal,springload/react-modal,ineo6/react-modal,rackt/react-modal,mgh/react-modal,rackt/react-modal,dminuoso/react-modal