update for runtime-go 1.18.0 (#696)

diff --git a/helm/openwhisk/runtimes.json b/helm/openwhisk/runtimes.json
index 8a9fb2a..fd14f69 100644
--- a/helm/openwhisk/runtimes.json
+++ b/helm/openwhisk/runtimes.json
@@ -213,7 +213,7 @@
                 "image": {
                     "prefix": "openwhisk",
                     "name": "action-golang-v1.15",
-                    "tag": "1.17.0"
+                    "tag": "1.18.0"
                 }
             }
         ],