8241144: Javadoc is not generated for new module jdk.nio.mapmode

Jdk.nio.mapmode is not present in DOCS_MODULES defined by Modules.gmk

Reviewed-by: erikj, ihse

make/common/Modules.gmk

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2014, 2019, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2014, 2020, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # This code is free software; you can redistribute it and/or modify it
@@ -171,6 +171,7 @@ DOCS_MODULES += \
     jdk.naming.dns \
     jdk.naming.rmi \
     jdk.net \
+    jdk.nio.mapmode \
     jdk.rmic \
     jdk.scripting.nashorn \
     jdk.sctp \