docs(contributing): correct push -f command
docs(contributing): correct push -f command

`git push -f` needs branch specification

In all cases, please consult the git manpages before consulting angular's contributing guide
when you need help with git, thx ^^

Closes #10356

ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js