Improve anagram problem statement (#1529)

- Add definition of anagram
- Improve readability

Fixes #1528

Author: gris

exercises/anagram/description.md

@@ -1,4 +1,5 @@
-Given a word and a list of possible anagrams, select the correct sublist.
+An anagram is a rearrangement of letters to form a new word.
+Given a word and a list of candidates, select the sublist of anagrams of the given word.
 
 Given `"listen"` and a list of candidates like `"enlists" "google"
 "inlets" "banana"` the program should return a list containing