Write a function that removes any non-letters from a string, returning a well-known film title.
letters_only("R!=:~0o0./c&}9k`60=y") ➞ "Rocky"
letters_only("^,]%4B|@56a![0{2m>b1&4i4") ➞ "Bambi"
letters_only("^U)6$22>8p).") ➞ "Up"See the Resources tab for more information on Ruby string methods.