'Improve' is one of a number of verbs that carry a similar (not identical) meaning when used intransitively in the active and transitively in the passive. In the active use, the 'causer' of the action is not mentioned, and may not even exist. In the passive use, the causer may not be mentioned, but does often exist.
Similar examples are:
The door opened/was opened.
The car is moving/being moved.
The cake is baking/being baked.
ps. Sorry. Both your sentences are fine.