Help: Update prefs ui file

Specify `Gui::FileChooser::Directory` mode for `Gui::PrefFileChooser` and update path in tooltips.

src/Mod/Help/dlgPreferencesHelp.ui

@@ -45,9 +45,9 @@ This is currently not available...</string>
         <property name="toolTip">
          <string>Set this to a custom URL or the folder where the help files are located.
 You can easily download the documentation for offline use by using the Addon
-Manager and installing the &quot;offline documentation&quot; addon. If this
+Manager and installing the &quot;offline-documentation&quot; addon. If this
 field is left blank, FreeCAD will automatically search for the help files at
-the default location ($USERAPPDATADIR/Mod/Documentation).</string>
+the default location ($USERAPPDATADIR/Mod/offline-documentation).</string>
         </property>
         <property name="text">
          <string>Custom location</string>
@@ -82,12 +82,15 @@ to get French translation of the documentation.</string>
       </item>
       <item row="6" column="1">
        <widget class="Gui::PrefFileChooser" name="fileChooser">
+        <property name="mode">
+         <enum>Gui::FileChooser::Directory</enum>
+        </property>
         <property name="toolTip">
          <string>Set this to a custom URL or the folder where the help files are located.
 You can easily download the documentation for offline use by using the Addon
-Manager and installing the &quot;offline documentation&quot; addon. If this
+Manager and installing the &quot;offline-documentation&quot; addon. If this
 field is left blank, FreeCAD will automatically search for the help files at
-the default location ($USERAPPDATADIR/Mod/Documentation).</string>
+the default location ($USERAPPDATADIR/Mod/offline-documentation).</string>
         </property>
         <property name="fileName">
          <string/>