Correct spelling, explanation: this form is the correct one as we add adverbial suffix -ly to the adjective true. e is skipped, perhaps due to pronunciation, but l is not doubled; there is no need to do so. Therefore truly is the correct spelling and trully is a mistake.

Definition of truly:
adverb, in a way that something is truth
Jenny truly believe she will win the competition.
Jack is truly considered the best player in the team.

Incorrect spelling, explanation: we do not spell this adverb with double l, because we add the suffix to the word true which doesn’t end with another l. Vowel e is omitted but l is not doubled. Because of this truly is the correct spelling and trully is not correct.

Incorrect spelling

Incorrect spelling

Incorrect spelling