docs: use open instead of opn in the README (#750)

The node module `opn` has been renamed to `open`, so I've changed the example to reflect that.

README.md

@@ -86,7 +86,7 @@ Let's take a look at a complete example.
 const {OAuth2Client} = require('google-auth-library');
 const http = require('http');
 const url = require('url');
-const opn = require('opn');
+const open = require('open');
 const destroyer = require('server-destroy');
 
 // Download your OAuth2 configuration from the Google
@@ -158,7 +158,7 @@ function getAuthenticatedClient() {
  })
  .listen(3000, () => {
  // open the browser to the authorize url to start the workflow
- opn(authorizeUrl, {wait: false}).then(cp => cp.unref());
+ open(authorizeUrl, {wait: false}).then(cp => cp.unref());
  });
  destroyer(server);
  });