docs: missing return in usage builder function (#2328)

docs/api.md

@@ -1685,7 +1685,7 @@ to provide configuration for the positional arguments accepted by your program:
 ```js
 const argv = require('yargs/yargs')(process.argv.slice(2))
   .usage('$0 <port>', 'start the application server', (yargs) => {
-    yargs.positional('port', {
+    return yargs.positional('port', {
       describe: 'the port that your application should bind to',
       type: 'number'
     })