Add required alt attribute to hero header image on homepage

Closes GH-693.

Author: BeardedFish

index.php

@@ -92,7 +92,7 @@
 
 $intro = <<<EOF
   <div class="hero">
-    <img class="hero-logo" src="/images/logos/php-logo-white.svg" width="240" height="120">
+    <img class="hero-logo" src="/images/logos/php-logo-white.svg" alt="php" width="240" height="120">
     <p class="hero-text">A <strong>popular general-purpose scripting language</strong> that is especially suited to web development.<br />Fast, flexible and pragmatic, PHP powers everything from your blog to the most popular websites in the world.</p>
     <div class="hero-actions">
       <a href="/releases/8.1/index.php" class="hero-btn hero-btn-primary">What's new in 8.1</a>