Word search should accept array of words

Currently, the word search method accepts a single string, it should be an array of strings:
```ruby
word = 'foo'
response = client.transcripts.word_search(
  transcript_id: transcript.id,
  words: word
)
```

It should be
```ruby
word = 'foo'
response = client.transcripts.word_search(
  transcript_id: transcript.id,
  words: [word]
)
```