Improve commit message example in CONTRIBUTING.md (#10357)

CONTRIBUTING.md

 
 # Example of a good commit message
 
-    Create a Valo icon font for icons used in Valo (#18472)
+    Create a Valo icon font for icons used in Valo
 
     Valo uses only a handful of icons from Font Awesome. This change introduces a separate icon font for valo (9KB instead of 80KB) and decouples Valo from Font Awesome to enable updating Font Awesome without taking Valo into account.
 
 
     For backwards compatibility, Font Awesome is loaded by default
 
+    Closes #123456
+
 ## Example breakdown
 
-Start with a good Commit message in imperative form. Reference a ticket number if applicable:
+Start with a good Commit message in imperative form.
 
-    Create a Valo icon font for icons used in Valo (#18472)
+    Create a Valo icon font for icons used in Valo
 
 ### Describe the problem:
 
 
     For backwards compatibility, Font Awesome is loaded by default
 
+### Reference an Issue:
+
+If applicable, reference an open issue that your pull request resolves as described in [this article](https://help.github.com/articles/closing-issues-using-keywords/). In the example given above this corresponds to the line
+
+    Closes #123456
+
+where `123456` is the hypothetical issue number.
+
 # Include a test
 
 Ideally, we would like all patches to include automated tests. Unit tests are preferred. If there’s a change to UI Code, we would prefer a TestBench test. If that’s not possible (you as a contributor lack a TestBench license), we can make do with a Test UI class that contains a test case as well a clear instructions for how to perform the test. This also goes for features that are hard to test automatically.