[gtkmm-documentation] Don’t write main() without a return type

[Daniel Boles <dboles src gnome org>]

commit 6ea7ef35b3df2ddaa74d979c0647f74985db3a1f
Author: Daniel Boles <dboles src gmail com>
Date:   Sun Jan 14 20:45:30 2018 +0000

    Don’t write main() without a return type
    
    Yikes! That’s not C++.

 docs/tutorial/C/index-in.docbook |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/docs/tutorial/C/index-in.docbook b/docs/tutorial/C/index-in.docbook
index 9dd8812..fc6789c 100644
--- a/docs/tutorial/C/index-in.docbook
+++ b/docs/tutorial/C/index-in.docbook
@@ -8299,7 +8299,7 @@ void on_button_clicked()
     std::cout &#60;&#60; "Hello World" &#60;&#60; std::endl;
 }
 
-main()
+int main()
 {
     Gtk::Button button("Hello World");
     button.signal_clicked().connect(sigc::ptr_fun(&amp;on_button_clicked));
@@ -8379,7 +8379,7 @@ class some_class
 
 some_class some_object;
 
-main()
+int main()
 {
     Gtk::Button button;
     button.signal_clicked().connect( sigc::ptr_fun(&amp;on_button_clicked) );