[gimp] Update bug.md

[Alexandre Prokoudine <aprokoudine src gnome org>]

commit 16cd3014c86eca554f2a321f82d32623b738ee23
Author: Alexandre Prokoudine <alexandre prokoudine gmail com>
Date:   Thu Aug 8 13:09:26 2019 +0000

    Update bug.md

 .gitlab/issue_templates/bug.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab/issue_templates/bug.md b/.gitlab/issue_templates/bug.md
index a1c124bc27..a0a24d2187 100644
--- a/.gitlab/issue_templates/bug.md
+++ b/.gitlab/issue_templates/bug.md
@@ -4,7 +4,7 @@ Note: bug reporters are expected to have verified the bug still exists
 either in the last stable version of GIMP or on updated development code
 (master branch).
 
-Operating System:
+Operating System: [Windows? macOS? Linux?]
 
 Package: [flatpak? Installer from gimp.org? If another installer, tell us where from]