docs: update CONTRIBUTING.md with an example of self-assigning an iss…

…ue. (#1506)

Closes #1505.

---------

Co-authored-by: David Li <li.davidm96@gmail.com>

CONTRIBUTING.md

@@ -278,6 +278,7 @@ Before opening a pull request:
 
 1. Please check if there is a corresponding issue (_and if not, please make one_).
 2. Assign the issue to yourself by commenting "take" in the issue.
+   (_Here's an [example](https://github.com/apache/arrow-adbc/issues/1505#issuecomment-1920134722)._)
 3. At the bottom of the PR description, add `Closes #NNNN` where `NNNN` is the
    issue number, so that the issue gets linked to your PR properly. ("Fixes"
    and other keywords that GitHub recognizes are also OK, of course.)