fix: replace commas with underscores in atomic-css (#122)

Author: matttrice

guide/faq.md

@@ -24,7 +24,7 @@ The second column
 Go further, you can customize the size of each column like:
 
 ```html
-<div class="grid grid-cols-[200px,1fr,10%] gap-4">
+<div class="grid grid-cols-[200px_1fr_10%] gap-4">
 <div>
 
 The first column (200px)