Fix Java 11 Javadoc URL

api/build.gradle

@@ -83,7 +83,7 @@ javadoc {
             'http://www.slf4j.org/apidocs/',
             'https://google.github.io/guava/releases/25.1-jre/api/docs/',
             'https://google.github.io/guice/api-docs/5.0.1/javadoc/',
-            'https://docs.oracle.com/javase/11/docs/api/',
+            'https://docs.oracle.com/en/java/javase/11/docs/api//',
             'https://jd.adventure.kyori.net/api/4.7.0/'
     )