Both are correct.
We have two ways of dealing with words ending in 'o'. Those that have been in the language for many hundreds of years, such as 'potato' and 'tomato', take an 'e' before the 's'. Those that are more recent imports into the language, and/or are still considered to be loan words from other languages, such as 'tango', form their plurals with just an 's'.
'Mango' seems to fall somewhere in between these groups, and both forms are acceptable.