Update openapi.yaml (#490)

Incorporate new prefix for x-issuer and x-jwks_uri flags. Stops gcloud throwing warnings when sample openapi.yaml is deployed. See #483

endpoints/getting-started/src/main/appengine/openapi.yaml

@@ -94,15 +94,15 @@ securityDefinitions:
     flow: "implicit"
     type: "oauth2"
     # This must match the 'iss' field in the JWT.
-    x-issuer: "jwt-client.endpoints.sample.google.com"
+    x-google-issuer: "jwt-client.endpoints.sample.google.com"
     # Update this with your service account's email address.
-    x-jwks_uri: "https://www.googleapis.com/service_accounts/v1/jwk/YOUR-SERVICE-ACCOUNT-EMAIL"
+    x-google-jwks_uri: "https://www.googleapis.com/service_accounts/v1/jwk/YOUR-SERVICE-ACCOUNT-EMAIL"
   # This section configures authentication using Google OAuth2 ID Tokens.
   # ID Tokens can be obtained using OAuth2 clients, and can be used to access
   # your API on behalf of a particular user.
   google_id_token:
     authorizationUrl: ""
     flow: "implicit"
     type: "oauth2"
-    x-issuer: "accounts.google.com"
-    x-jwks_uri: "https://www.googleapis.com/oauth2/v1/certs"
+    x-google-issuer: "accounts.google.com"
+    x-google-jwks_uri: "https://www.googleapis.com/oauth2/v1/certs"