[iagno] Add missing mnemonic.

[Arnaud B. <arnaudb src gnome org>]

commit be74d7e06b35c0323eb057feaf9c980c712286fb
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Mon Dec 9 18:24:11 2019 +0100

    Add missing mnemonic.

 src/iagno.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/iagno.vala b/src/iagno.vala
index bee15c7..1f58844 100644
--- a/src/iagno.vala
+++ b/src/iagno.vala
@@ -408,7 +408,7 @@ private class Iagno : Gtk.Application, BaseApplication
         window = new GameWindow ("/org/gnome/Reversi/ui/iagno.css",
                                  PROGRAM_NAME,
                                  /* Translators: hamburger menu entry; open about dialog (with a mnemonic 
that appears pressing Alt) */
-                                 _("About Reversi"),
+                                 _("_About Reversi"),
                                  start_now,
                                  GameWindowFlags.SHOW_START_BUTTON
                                  | GameWindowFlags.HAS_SOUND