UI: Fix custom ffmpeg output file / URL entry

The default value in the .ui file was out of sync so the signal to
update the stacked widget was never sent.

Fixes https://obsproject.com/mantis/view.php?id=1060

UI/forms/OBSBasicSettings.ui

@@ -1878,7 +1878,7 @@
                          <item row="1" column="1">
                           <widget class="QStackedWidget" name="stackedWidget_2">
                            <property name="currentIndex">
-                            <number>1</number>
+                            <number>0</number>
                            </property>
                            <widget class="QWidget" name="page_5">
                             <layout class="QHBoxLayout" name="horizontalLayout_12">