update confusing usage example in readme

Example assumed that component yields a hash before but a component after
changing syntax. Since angle bracket invocation does not affect yielded
value it should be the same before and after.

[ci skip]

README.md

@@ -24,9 +24,9 @@ but here are a few examples of "before"/"after" to whet your appetite:
 ```hbs
 {{site-header user=this.user class=(if this.user.isAdmin "admin")}}
 
-{{#super-select selected=this.user.country as |s|}}
+{{#super-select selected=this.user.country as |option|}}
   {{#each this.availableCountries as |country|}}
-    {{#s.option value=country}}{{country.name}}{{/s.option}}
+    {{#option value=country}}{{country.name}}{{/s.option}}
   {{/each}}
 {{/super-select}}
 ```