Use airflow_version in subject (#62)

dev/README_RELEASE_CLIENT.md

diff --git a/dev/README_RELEASE_CLIENT.md b/dev/README_RELEASE_CLIENT.md
index 902b966..43ea1aa 100644
--- a/dev/README_RELEASE_CLIENT.md
+++ b/dev/README_RELEASE_CLIENT.md
@@ -213,7 +213,7 @@
 
 ```shell script
 cat <<EOF
-[VOTE] Release Airflow Python Client ${VERSION_WITHOUT_RC} from ${VERSION}
+[VOTE] Release Airflow Python Client ${VERSION_WITHOUT_RC} from Airflow ${AIRFLOW_VERSION}
 EOF
 ```