Fix URL of the og:image

Author: jsmitka

_upcoming/2026/index.html

@@ -12,7 +12,7 @@
     <meta property="og:url" content="https://cz.pycon.org/2026/">
     <meta property="og:title" content="PyCon CZ 26, September 2026, Pilsen">
     <meta property="og:description" content="Join us for 3 days full of Python and fun!!!">
-    <meta property="og:image" content="https://cz.pycon.org/2026/static/social-media-general.png">
+    <meta property="og:image" content="https://cz.pycon.org/2026/static/social-media-general.jpg">
     <meta property="og:image:width" content="2400">
     <meta property="og:image:height" content="1260">
     <meta name="twitter:card" content="summary_large_image">