RFR: JDK 11 P1: 8209806: API docs should be updated to refer to javase11

[Jonathan Gibbons]

Please review a simple high priority fix for JDK 11 that updates the 
link in the bottom of each page of the generated API docs.

Instead of simply changing "10" to "11", the fix is to use the macro 
$(VERSION_NUMBER) instead, in line with other URLs nearby in the same 
makefile, and in the ExtLink taglet [1]

JBS: https://bugs.openjdk.java.net/browse/JDK-8209806
Webrev: http://cr.openjdk.java.net/~jjg/8209806/webrev.00/index.html

[1]: 
http://hg.openjdk.java.net/jdk/jdk11/file/ed52ea83f830/make/jdk/src/classes/build/tools/taglet/ExtLink.java#l66

-- Jon