[Elixi] Fix hints for all exercises

[angelikatyborska]

I noticed that the guessing game's hints don't work, so I went and reviewed all the hint files:

• Make sure every hint section is a list
• Remove nested lists, they do not render on exercism.lol
• Make sure every hint consists of one or more sentences (capitalized, ends with a dot).

I didn't double-check if the hints' headers are the same as in introduction. I hope somebody else will make a tool for that soon.