Tweak TileMap editor's missing source message to use "bottom panel" term

This term is used throughout the editor (including EditorPlugin)
as these bottom panels are technically not tabs.

editor/plugins/tiles/tile_map_layer_editor.cpp

@@ -2363,7 +2363,7 @@ TileMapLayerEditorTilesPlugin::TileMapLayerEditorTilesPlugin() {
 	tiles_bottom_panel->set_name(TTR("Tiles"));
 
 	missing_source_label = memnew(Label);
-	missing_source_label->set_text(TTR("This TileMap's TileSet has no source configured. Go to the TileSet bottom tab to add one."));
+	missing_source_label->set_text(TTR("This TileMap's TileSet has no source configured. Go to the TileSet bottom panel to add one."));
 	missing_source_label->set_h_size_flags(Control::SIZE_EXPAND_FILL);
 	missing_source_label->set_v_size_flags(Control::SIZE_EXPAND_FILL);
 	missing_source_label->set_horizontal_alignment(HORIZONTAL_ALIGNMENT_CENTER);