Update slug to match the filename

I ran this -

`for file in app/support/stagecraft_stub/responses/**/*.json; do SLUG=$(cat $file | jq .slug | sed s/\"//g); NAME=$(basename $file); if [[ "${NAME%.*}" != $SLUG ]]; then echo ${NAME%.*}; echo $file; echo $SLUG; fi; done`

And all other filenames matched the slug. This also isnt a valid slug,
so fails the import
Merge pull request #792 from alphagov/rename-hmrc-prototype

Update slug to match the filename