Fix logo srcset for 3x

Author: dhalbert

_includes/header.html

@@ -29,7 +29,7 @@
             <img src="{{ "/assets/images/logo-dark.png" | relative_url }}"
                  srcset="{{ "/assets/images/logo-dark.png" | relative_url }} 1x,
                          {{ "/assets/images/[email protected]" | relative_url }} 2x,
-                         {{ "/assets/images/logo-dark3x.png" | relative_url }} 3x"
+                         {{ "/assets/images/logo-dark@3x.png" | relative_url }} 3x"
                                          alt="CircuitPython Logo"
                                          height="60" width="136">
           </a>