Fix header image for mobile sceens (#1900)

Author: kylemclaren

index.html.md

@@ -7,7 +7,7 @@ nav: firecracker
 ---
 
 <figure>
-  <img src="/static/images/doc-main.png" alt="Illustration by Annie Ruygt of Frankie the hot air balloon waving to a bird sitting on a hour roof" class="max-w-lg">
+  <img src="/static/images/doc-main.png" alt="Illustration by Annie Ruygt of Frankie the hot air balloon waving to a bird sitting on a hour roof" class="w-full max-w-lg mx-auto">
 </figure>
 
 <div class="index-page">