CONTRIBUTING: fix repo reference rendering

Minor change to wrap repository references in code blocks to ensure
it renders as written on the GitHub web view. The documentation
is confusing as GitHub elides the full repository name in the
reference.

Change-Id: I173aabdda21061cfaf5cdf3d0bd239b0e4eb4af2
Signed-off-by: Cedric Charly <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1172006
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: cedricgc

CONTRIBUTING.md

@@ -616,22 +616,22 @@ markup language.
 
 ### Referencing issues
 
-The special notation "Fixes #12345" associates the change with issue 12345 in
+The special notation `Fixes #12345` associates the change with issue 12345 in
 the [CUE issue tracker](https://cuelang.org/issue/12345) When this change is
 eventually applied, the issue tracker will automatically mark the issue as
 fixed.
 
 
 If the change is a partial step towards the resolution of the issue, uses the
-notation "Updates #12345".  This will leave a comment in the issue linking back
+notation `Updates #12345`.  This will leave a comment in the issue linking back
 to the change in Gerrit, but it will not close the issue when the change is
 applied.
 
 
 All issues are tracked in the main repository's issue tracker.
 If you are sending a change against a subrepository, you must use the
 fully-qualified syntax supported by GitHub to make sure the change is linked to
-the issue in the main repository, not the subrepository (eg. "Fixes cue-lang/cue#999").
+the issue in the main repository, not the subrepository (eg. `Fixes cue-lang/cue#999`).