fixed typo

mod/upgrade.txt

@@ -37,7 +37,7 @@ required changes in code:
 * remove '_utf8' from lang pack names, use new {a} syntax
 * replace helps with new 'xxx_hlp' strings
 * please note the $plugin->requires in version.php has to be bigger than 2010000000,
-  otherwise the pluguin is marked as outdated and upgrade is interrupted
+  otherwise the plugin is marked as outdated and upgrade is interrupted
 
 optional - no changes needed in older code:
 * settingstree.php replaced by settings.php - just unset the $settings if you want to make custom part of settings admin tree