Update word count solution filename and ns

Hi, I had to make some changes to `word-count/word_count_test.clj` last night to be able to run the tests via a REPL and I thought it would be worthwhile submitting a PR to avoid others having to do the same.

I believe it's convention for filenames to use underscores instead of hyphens, I've also updated the referenced `ns` in `word-count/word_count_test.clj` to be `word-count` instead of `phrase` for consistency.