Minor changes to homepage

- Change subtitle to "Planta 8 - Tardor 2022"
- Center title and subtitle in the mobile view

Change-Id: Ifb81f0317898464ee69780388fb38aa0c70e6d79
diff --git a/index.php b/index.php
index 1b87a7d..65baf23 100644
--- a/index.php
+++ b/index.php
@@ -35,7 +35,7 @@
 				<div class="header-container">
 					<header>
 						<h2>Carrot Cake</h2>
-						<h3>Facultat de Física - Tardor 2022</h3>
+						<h3>Planta 8 - Tardor 2022</h3>
 					</header>
 					<img src="/bin/images/vuit.png" class="logo">
 				</div>