better example for tokenizer lambda [ci skip]

Splitting on whitespace makes more sense in the context of counting words in an essay.

To elaborate, consider how the Unix `wc` command works
```
$ echo "Rail's tokenizer-example" | wc -w
2
```
vs how this example tokenizer would behave
```
irb(main):001:0> "Rail's tokenizer-example".scan(/\w+/).length
=> 4
```