Javadoc fix.

The use of `_` as an identifier is discouraged in the JLS, therefor do
not suggest its use in our javadoc.

ref:
http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.27.1

> It is a compile-time error if a lambda parameter has the name _ (that
is, a single underscore character).

> The use of the variable name _ in any context is discouraged. Future
versions of the Java programming language may reserve this name as a
keyword and/or give it special semantics.