Update extending-api.md
Update extending-api.md

After proofreading.
pkarw/vue-storefront,pkarw/vue-storefront,pkarw/vue-storefront,DivanteLtd/vue-storefront,DivanteLtd/vue-storefront,pkarw/vue-storefront,DivanteLtd/vue-storefront,DivanteLtd/vue-storefront