Update copyright in About dialog (#603)

Author: kaiw

meld/resources/ui/about-dialog.ui

@@ -6,7 +6,7 @@
     <property name="can_focus">False</property>
     <property name="type_hint">dialog</property>
     <property name="copyright" translatable="yes">Copyright © 2002-2009 Stephen Kennedy
-Copyright © 2009-2019 Kai Willadsen</property>
+Copyright © 2009-2021 Kai Willadsen</property>
     <property name="website">https://meldmerge.org/</property>
     <property name="authors">Stephen Kennedy
 Kai Willadsen