Replace 'git whatchanged' with 'git log --raw' as the former is deprecated

Author: Bill-Sommerfeld

docs/contributing/index.md

@@ -268,7 +268,7 @@ Your RTI e-mail should include:
   https://illumos.org/issues/10052
 * The changes that were reviewed; e.g., a link to your [Gerrit](./gerrit)
   review, or an attached patch otherwise.
-* The full "change set description" (i.e., `git whatchanged -v origin/master..`)
+* The full "change set description" (i.e., `git log --raw -v origin/master..`)
   including:
     * Issue number(s) and description(s)
     * `Reviewed by: First Last <first.last@example.com>` lines