SpellChecker : SpellChecker.indexDictionary : Javadoc incorrect [LUCENE-4487]

example code in javadoc seems to be incorrect

code changes from #4631

means the javadoc

```Java
SpellChecker spellchecker = new SpellChecker(spellIndexDirectory);
// To index a field of a user index:
spellchecker.indexDictionary(new LuceneDictionary(my_lucene_reader, a_field));
// To index a file containing words:
spellchecker.indexDictionary(new PlainTextDictionary(new File("myfile.txt")));
String[] suggestions = spellchecker.suggestSimilar("misspelt", 5);
```

is incorrect i think something like this is better maybe

```Java
SpellChecker spellchecker = new SpellChecker(spellIndexDirectory);
// To index a field of a user index:
spellchecker.indexDictionary(new LuceneDictionary(my_lucene_reader, a_field), new IndexWriterConfig(Version.LUCENE_CURRENT, null), false);
// To index a file containing words:
spellchecker.indexDictionary(new PlainTextDictionary(new File("myfile.txt")), new IndexWriterConfig(Version.LUCENE_CURRENT, null), false);
String[] suggestions = spellchecker.suggestSimilar("misspelt", 5);
```

I'd possibly add something about creating a spellIndexDirectory too, not clear

```Java
Directory spellIndexDirectory = FSDirectory.open("/dev/tmp/SOME_WORKING_DIR");
SpellChecker spellchecker = new SpellChecker(spellIndexDirectory);
// To index a field of a user index:
spellchecker.indexDictionary(new LuceneDictionary(my_lucene_reader, a_field), new IndexWriterConfig(Version.LUCENE_CURRENT, null), false);
// To index a file containing words:
spellchecker.indexDictionary(new PlainTextDictionary(new File("myfile.txt")), new IndexWriterConfig(Version.LUCENE_CURRENT, null), false);
String[] suggestions = spellchecker.suggestSimilar("misspelt", 5);
```

would of sent a patch but not familiar with how to get access to the apache git repo todo a pull/push

cheers

Ant

---
Migrated from [LUCENE-4487](https://issues.apache.org/jira/browse/LUCENE-4487) by Anthony Mckale
Linked issues:
- #4631

Start with the SpellChecker.indexDictionary Javadoc and its current method signature, then compare the example with the API change from #4631. Update the example so its calls and setup match the available API, and verify that the generated Javadoc example is accurate.

Written by the indexing model from the issue text.