Gui: "Default unit system" in title case in prefs

Author: Roy-043

src/Gui/PreferencePages/DlgSettingsGeneral.ui

@@ -52,7 +52,7 @@
        <item row="1" column="0">
         <widget class="QLabel" name="unitSystemLabel">
          <property name="text">
-          <string>Default Unit system:</string>
+          <string>Default unit system:</string>
          </property>
         </widget>
        </item>