Fixes PHP 8.1 E_DEPRECATED in GitHub Template (#23)

If no longDescription, `null` is passed to `str_replace()`,
which is deprecated now in PHP 8.  Casting value to
string to avoid the E_DEPRECATED.

Author: alegag

tpl/github.class.phtml

@@ -55,7 +55,7 @@
 
 <?= $info->shortDescription ?> 
 
-<?= str_replace("\n", "  \n", $info->longDescription) ?> 
+<?= str_replace("\n", "  \n", (string)$info->longDescription) ?> 
 
 **Parameters**