Typo fix docs-demo -> docs-hero (ember-learn#396)

Author: bantic

tests/dummy/app/pods/docs/components/docs-hero/template.md

@@ -7,7 +7,7 @@ The component has three arguments. By default, these arguments are pulled from y
   - `prefix` and `heading` come from the `name` field
   - `byline` comes from the `description` field
 
-As long as these are present, you can just render `{{docs-demo}}` with no arguments:
+As long as these are present, you can just render `{{docs-hero}}` with no arguments:
 
 {{docs/components/docs-hero/demo-1}}