True, accidentally is the correct spelling. The latter one is also accepted but I don't think it's widely accepted in formal writing.
Dumbtionary (a dictionary of misspelled words) explains it:
The word listed above (accidentally) is probably the correct spelling for the word that you entered (accidently). This is just an educated guess based on commonly misspelled words. To double check that this is the correct word you can use the resources below to find the definition of accidentally, antonyms for accidentally, synonyms for accidentally, quotes relating to accidentally and other information about accidentally.
And the sources given there validates the former spelling as the correct one.
Furthermore, the word accidently returned no results on OxofordDictionaries.com. I also tried with option of 'British and World English' and the 'US English'. Zero result which means it's not a BrE or an AmE stuff.