Small refactoring of example.clj for anagram

`frequencies` is a core function that will make the word into a
map of letter frequencies, e.g.:

(canonicalize "hello")
;=> {\h 1, \e 1, \l 2, \o 1}

`frequencies` seems a bit clearer than the `split #""` then `sort`.

(Interestingly, the `split` is not necessary even if `sort`ing, since
Clojure will represent `word` as an Object array of Characters,
then return it sorted. `frequencies` probably ends up slower than `sort`
over large strings due to the incremental object allocation, but also
is likely _faster_ than `split` ... `sort`.)
Small clarification for anagram README

It wasn't clear to me whether this was supposed to return a list of one
vs. a single string in the case of one match, and an empty list vs.
nil/null/() in the case of no matches.