Tell user to supply BOTH keys in .env.development. (#28405)
Tell user to supply BOTH keys in .env.development. (#28405)

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"?

With this one-liner we can avoid newcomers being put off by an ambiguous instruction being given in tutorial codeblocks.
gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby