Fix outdated links to shopify API docs (bold-commerce#325)

[rahji]

I replaced the broken links using the following one-liner. Also updated a typo for a link in variant.go

for f in `ls *.go`; do
perl -pi.bak -e 's!help.shopify.com/.*/(\w+)!shopify.dev/docs/api/admin-rest/latest/resources/$1!' $f
done

[oliver006]

That's awesome, thanks for doing this so quickly and thanks for the PR!

I'll review and merge later.