RFR (XS): 8231161: Wrong return type in code sample in Collector API documentation

[Julia Boes]

Hi,

This is a minor fix of the Collector class-level documentation, where 
the wrong type declaration was used in a code sample.

While at it, I added a compilation test for all code samples of this 
class-level javadoc.


Bug: https://bugs.openjdk.java.net/browse/JDK-8231161

Webrev: 
http://cr.openjdk.java.net/~jboes/webrevs/8231161/webrev.00/index.html


Regards,

Julia