fix a bit awkward UI text

Change-Id: I3c58e329a43e9117fe2be751442bd9043b758e41

cui/uiconfig/ui/optsecuritypage.ui

@@ -33,7 +33,7 @@
                     <property name="can_focus">False</property>
                     <property name="hexpand">True</property>
                     <property name="xalign">0</property>
-                    <property name="label" translatable="yes">Maintain a list of Time Stamping Authority (TSA) URLs to be optionally for digital signatures in PDF export.</property>
+                    <property name="label" translatable="yes">Maintain a list of Time Stamping Authority (TSA) URLs to be used for digital signatures in PDF export.</property>
                     <property name="wrap">True</property>
                     <property name="max_width_chars">56</property>
                   </object>