[gnome-calendar] docs: Fix file name in HACKING guide

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 2452ea07fb44eacfea03f5c4e89f91a0392c5d28
Author: Felix Eckhofer <felix eckhofer com>
Date:   Wed Mar 20 13:01:11 2019 +0100

    docs: Fix file name in HACKING guide

 HACKING.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/HACKING.md b/HACKING.md
index 6f9b8689..618ab5fb 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -134,5 +134,5 @@ foo_bar_do_somthing_sync (a, 1, object, data,
 
 # Contributing guidelines
 
-See CONTRIBUTIONS.md file for the contribution guidelines, and the Code of Conduct
+See CONTRIBUTING.md file for the contribution guidelines, and the Code of Conduct
 that contributors are expected to follow.