[gtkmm-documentation] Fix the build.

[Murray Cumming <murrayc src gnome org>]

commit 00b604738ef7355eedfff2eedafca411b143512c
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Jul 26 12:41:59 2016 +0200

    Fix the build.

 .../others/cellrenderercustom/cellrendererpopup.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/examples/others/cellrenderercustom/cellrendererpopup.cc 
b/examples/others/cellrenderercustom/cellrendererpopup.cc
index 3f4838a..2f59e59 100644
--- a/examples/others/cellrenderercustom/cellrendererpopup.cc
+++ b/examples/others/cellrenderercustom/cellrendererpopup.cc
@@ -127,7 +127,7 @@ Gtk::CellEditable* CellRendererPopup::start_editing_vfunc(GdkEvent*,
   if(!property_editable())
     return nullptr;
 
-  auto popup_entry = std::make_unique<PopupEntry>(path));
+  auto popup_entry = std::make_unique<PopupEntry>(path);
 
   popup_entry->signal_editing_done ().connect(sigc::mem_fun(*this, &Self::on_popup_editing_done));
   popup_entry->signal_arrow_clicked().connect(sigc::mem_fun(*this, &Self::on_popup_arrow_clicked));