Both are being used and neither is incorrect. “An historic” used to be more prevalent in British English until around 1990, but “a historic” has become the standard choice now: <a href="https://books.google.com/ngrams/graph?content=a+historic%2Can+historic&year_start=1800&year_end=2022&corpus=en&smoothing=3&case_insensitive=false" rel="nofollow">https://books.google.com/ngrams/graph?content=a+historic%2Ca...
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