Fix calculation of expiration timestamp

`currentTimestamp` is in millis already which is why it should not be mulitplied by 1000.

This bug was introduced in bba32e0.

src/main/java/org/jenkinsci/plugins/oic/OicCredentials.java

@@ -48,7 +48,7 @@ public OicCredentials(
 
         if (expiresInSeconds != null && expiresInSeconds > 0) {
             long allowedClockSkewFixed = Util.fixNull(allowedClockSkewSeconds, 60L);
-            this.expiresAtMillis = (currentTimestamp + expiresInSeconds + allowedClockSkewFixed) * 1000;
+            this.expiresAtMillis = currentTimestamp + (expiresInSeconds + allowedClockSkewFixed) * 1000;
         } else {
             this.expiresAtMillis = null;
         }