In this case, it's not the accusative, but the genitive (Wessen Behebung? or Behebung wessen?).
That's the reason why ein changed into eines (note that Fehler needs to transform into Fehlers for this => "Behebung eines kleinen Fehlers").
Note, that this sentence uses the Nominalstil (nominal style), it transforms the verb (beheben) into a noun (Behebung). That's because there was no subject given in the original sentence. A correct version would be "I fixed a small error", it would literally translate into "Ich behob einen kleinen Fehler".
An alternative to translate "fixed a small error", would be "kleinen Fehler behoben", which is not a full sentence for its own, but your original english sentence is neither. It's only suitable for the use in lists, like changelogs.