[gnome-control-center] docs: Fix code block

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

commit 057610e982adf07a14a9ff96c9730acc2db4bd6e
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Sun May 13 19:45:52 2018 +0000

    docs: Fix code block

 docs/HACKING.md | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/docs/HACKING.md b/docs/HACKING.md
index 8415c2e3d..cf9265f50 100644
--- a/docs/HACKING.md
+++ b/docs/HACKING.md
@@ -21,6 +21,7 @@ Comment blocks should be formatted as following:
  * but have the closing slash a line after. Every line starts with
  * an asterisk that is aligned with every the rest of the block.
  */
+```
 
 ## Header (.h) files