Add db-migrator image to release notes

I forgot to add the DB Migrator image to the release notes.

Issue-ID: POLICY-3790
Change-Id: I922e578a2ab29a18bd7fd192ff199336312d014a
Signed-off-by: liamfallon <liam.fallon@est.tech>
diff --git a/docs/release-notes.rst b/docs/release-notes.rst
index d6f9ab0..f2751b6 100644
--- a/docs/release-notes.rst
+++ b/docs/release-notes.rst
@@ -44,6 +44,7 @@
      - 2.3.1
      - | onap/policy-jdk-alpine:2.3.1
        | onap/policy-jre-alpine:2.3.1
+       | onap/onap/policy-db-migrator:2.3.1
    * - policy/common
      - 1.9.1
      - N/A