Correct a couple of small grammar errors. (vercel#16221)

Just noticed a couple of tiny grammar errors.

docs/advanced-features/preview-mode.md

@@ -28,7 +28,7 @@ In the [Pages documentation](/docs/basic-features/pages.md) and the [Data Fetchi
 
 Static Generation is useful when your pages fetch data from a headless CMS. However, it’s not ideal when you’re writing a draft on your headless CMS and want to **preview** the draft immediately on your page. You’d want Next.js to render these pages at **request time** instead of build time and fetch the draft content instead of the published content. You’d want Next.js to bypass Static Generation only for this specific case.
 
-Next.js has the feature called **Preview Mode** which solves this problem. Here’s an instruction on how to use it.
+Next.js has a feature called **Preview Mode** which solves this problem. Here are instructions on how to use it.
 
 ## Step 1. Create and access a preview API route