Invalid example in SpellChecker javadoc [LUCENE-7971]

Hello, in Lucene javadoc for SpellChecker class there is an example of how to use it, however it's incorrect (wrong method signature).

File: lucene/suggest/src/java/org/apache/lucene/search/spell/SpellChecker.java

It should be:

```java
SpellChecker spellchecker = new SpellChecker(spellIndexDirectory);
IndexWriterConfig config = new IndexWriterConfig();
spellchecker.indexDictionary(new LuceneDictionary(my_lucene_reader, a_field), config, true);
spellchecker.indexDictionary(new PlainTextDictionary(new File("myfile.txt")), config, true);
String[] suggestions = spellchecker.suggestSimilar("misspelt", 5);
```

Instead of:

```java
SpellChecker spellchecker = new SpellChecker(spellIndexDirectory);
spellchecker.indexDictionary(new LuceneDictionary(my_lucene_reader, a_field));
spellchecker.indexDictionary(new PlainTextDictionary(new File("myfile.txt")));
String[] suggestions = spellchecker.suggestSimilar("misspelt", 5);
```

I'm in the middle of sending pull request on Github. Just wanted to create JIRA ticket so I can reference it in my pull request.

---
Migrated from [LUCENE-7971](https://issues.apache.org/jira/browse/LUCENE-7971) by Bartosz Firyn (@sarxos)

Open lucene/suggest/src/java/org/apache/lucene/search/spell/SpellChecker.java and find the javadoc usage example. Compare its indexDictionary calls with the signatures shown in the issue, then update the example so it matches the current API. Done means the javadoc presents the corrected calls and remains valid documentation.

Written by the indexing model from the issue text.