Fix syntax errors in Standalone code example

docs/source/index.mdx

@@ -139,23 +139,23 @@ const link = new HttpLink({ uri });
 
 const operation = {
   query: gql`query { hello }`,
-  variables: {} //optional
-  operationName: {} //optional
-  context: {} //optional
-  extensions: {} //optional
+  variables: {}, //optional
+  operationName: {}, //optional
+  context: {}, //optional
+  extensions: {}, //optional
 };
 
 // execute returns an Observable so it can be subscribed to
 execute(link, operation).subscribe({
   next: data => console.log(`received data: ${JSON.stringify(data, null, 2)}`),
   error: error => console.log(`received error ${error}`),
   complete: () => console.log(`complete`),
-})
+});
 
 // For single execution operations, a Promise can be used
 makePromise(execute(link, operation))
   .then(data => console.log(`received data ${JSON.stringify(data, null, 2)}`))
-  .catch(error => console.log(`received error ${error}`))
+  .catch(error => console.log(`received error ${error}`));
 ```
 
 > Note: to run Apollo Link on the server, ensure that you install `node-fetch` and pass its default export to as the `fetch` parameter to `HttpLink`