Create a function that takes a string and returns a string with its letters in alphabetical order.
alphabet_soup("hello") ➞ "ehllo"
alphabet_soup("daiebt") ➞ "abdeit"
alphabet_soup("hacker") ➞ "acehkr"
alphabet_soup("geek") ➞ "eegk"
alphabet_soup("javascript") ➞ "aacijprstv"
You can assume numbers and punctuation symbols won't be included in test cases. You'll only have to deal with single word, alphabetic characters.