Fix timezone serialization.

src/main/kotlin/ch/memobase/helpers/Activity.kt

@@ -66,7 +66,7 @@ object Activity {
     private fun now(): Literal {
         return ResourceFactory.createTypedLiteral(
             ZonedDateTime.now(ZoneId.of("UTC")).format(
-                DateTimeFormatter.ofPattern("YYYY-MM-dd'T'HH:mm:ssZ")
+                DateTimeFormatter.ofPattern("YYYY-MM-dd'T'HH:mm:ss'Z'")
             ).toString(), XSDDateTimeType("dateTime")
         )
     }