Update hero CTA to mention both PDF and EPUB

Co-authored-by: Ona <no-reply@ona.com>

Author: Siddhant-K-code

src/pages/index.astro

@@ -60,7 +60,7 @@ const totalChapters = allChapters.filter((c) => c.chapter).length;
               <line x1="16" y1="17" x2="8" y2="17" />
               <polyline points="10 9 9 9 8 9" />
             </svg>
-            Grab the PDF
+            Grab the PDF & EPUB
           </a>
           <a
             href="https://github.com/Siddhant-K-code/agentic-engineering-guide"