[fix][slack] fix typo in slackNotification func

```
groovy.lang.MissingPropertyException: No such property: toLowerCase for class: java.lang.String

```
    us: https://mirantis.jira.com/browse/PRODX-3254

Change-Id: I80643deb85dcd61ade65e176c8832bd8370fb9bb
diff --git a/src/com/mirantis/mcp/SlackNotification.groovy b/src/com/mirantis/mcp/SlackNotification.groovy
index 12d6825..8aa0cfe 100644
--- a/src/com/mirantis/mcp/SlackNotification.groovy
+++ b/src/com/mirantis/mcp/SlackNotification.groovy
@@ -45,7 +45,7 @@
     cred = common.getCredentialsById(credentialsId)
     hook_url_parsed = cred.getSecret().toString()
 
-    switch(buildStatusParam.toLowerCase) {
+    switch(buildStatusParam.toLowerCase()) {
         case "success":
             colorCode = "#00FF00"
             colorName = "green"