Update PULL_REQUEST_TEMPLATE.md
Update PULL_REQUEST_TEMPLATE.md

Use `closes` keyword, which will link PR with issue and close it as soon as code will be merged to master branch
oliviertassinari/material-ui,rscnt/material-ui,rscnt/material-ui,dmtrKovalenko/material-ui-pickers,mbrookes/material-ui,rscnt/material-ui,callemall/material-ui,callemall/material-ui,oliviertassinari/material-ui,callemall/material-ui,mui-org/material-ui,mbrookes/material-ui,mui-org/material-ui,mbrookes/material-ui,callemall/material-ui,oliviertassinari/material-ui,dmtrKovalenko/material-ui-pickers,mui-org/material-ui