CDAP-460: Updated docs to use new configurer style API for MapReduce

No mentions of ResourceSpecification or ServiceSpecification.Builder in the docs, so I only have MapReduceSpecification.Builder changes in this PR:

```
$ find . -name *.rst | xargs grep "ResourceSpecification"
$ find . -name *.rst | xargs grep "ServiceSpecification"
$ find . -name *.rst | xargs grep "MapReduceSpecification"
./cdap-docs/developers-manual/source/building-blocks/mapreduce-jobs.rst:       public MapReduceSpecification configure() {
./cdap-docs/developers-manual/source/building-blocks/mapreduce-jobs.rst:         return MapReduceSpecification.Builder.with()
```