add missing colon in Slide Transitions sidebar deck

Change-Id: I71d20c8e99b47b44220233af8bb81696bc3e6011
Reviewed-on: https://gerrit.libreoffice.org/21968Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sd/uiconfig/simpress/ui/slidetransitionspanel.ui

@@ -138,7 +138,7 @@
                       <object class="GtkLabel" id="variant_label">
                         <property name="visible">True</property>
                         <property name="can_focus">False</property>
-                        <property name="label" translatable="yes">Variant</property>
+                        <property name="label" translatable="yes">Variant:</property>
                       </object>
                       <packing>
                         <property name="left_attach">0</property>