[MINOR][DOCS] Updates to the Accumulator example in the programming guide. Fixed typos, AccumulatorV2 in Java

## What changes were proposed in this pull request?

This pull request contains updates to Scala and Java Accumulator code snippets in the programming guide.

- For Scala, the pull request fixes the signature of the 'add()' method in the custom Accumulator, which contained two params (as the old AccumulatorParam) instead of one (as in AccumulatorV2).

- The Java example was updated to use the AccumulatorV2 class since AccumulatorParam is marked as deprecated.

- Scala and Java examples are more consistent now.

## How was this patch tested?

This patch was tested manually by building the docs locally.

![image](https://cloud.githubusercontent.com/assets/6235869/20652099/77d98d18-b4f3-11e6-8565-a995fe8cf8e5.png)

Author: aokolnychyi <okolnychyyanton@gmail.com>

Closes #16024 from aokolnychyi/fixed_accumulator_example.