Gatsby E-commerce Tutorial should say to include STRIPE_PUBLISHABLE_KEY

[olisteadman]

Description

Tutorial codeblock indicates config should include STRIPE_SECRET_KEY but does not mention STRIPE_PUBLISHABLE_KEY.

For users supplying only the STRIPE_SECRET_KEY (without also adding STRIPE_PUBLISHABLE_KEY), gatsby-source-stripe fails to load allStripePrice as a field to be tested in GraphiQL; a browser pointed at http://localhost:8000 responds with:
Error: There was an error in your GraphQL query: Cannot query field "allStripePrice" on type "Query". Did you mean "allSitePage"?

We can avoid newcomers being put off by an ambiguous instruction being given in tutorial codeblocks, with a one-liner addition to https://github.com/gatsbyjs/gatsby/blob/master/docs/tutorial/ecommerce-tutorial/index.md