Of course, this should be derring-do. But the misspelling is understandable given the meaning of the word and its origins.
The word originated from the late Middle English dorryng do (meaning ‘daring to do’) being misprinted as derrynge do in the 16th-century editions of John Lydgate’s Chronicle of Troy.
The modern usage and spelling is usually most associated with Sir Walter Scott’s Ivanhoe.
*I would love to proofread a story where derring-do doesn’t look out of place. I am imagining pirates. Or an old-fashioned superhero.