finatra: fix ExceptionMapper documentation
finatra: fix ExceptionMapper documentation

Problem

The documentation refers to a non-existent `handle` method on `ExceptionMapper`
yet the method is called `handleException`.

Solution

This patch fixes said documentation to use the correct method name.

Differential Revision: https://phabricator.twitter.biz/D178159

twitter/finatra,twitter/finatra,twitter/finatra