doc: Remove unused parameter in Future conversion example
doc: Remove unused parameter in Future conversion example

Problem

The asScala method in the first example of the Futures documentation
passes in an unused parameter, implicit e: ExecutionContext.

Solution

Removed unused parameter.

Github issue #246

Signed-off-by: Yufan Gong <yufang@twitter.com>

Differential Revision: https://phabricator.twitter.biz/D301848

twitter/util,twitter/util,twitter/util