This can be stated in active form. AT: "Jesus saw the sky open" or "God opened the heavens to Jesus" (See: [[rc://en/ta/man/translate/figs-activepassive]])
Possible meanings are 1) this is simply a statement that the Spirit was in the form of a dove or 2) this is a simile that compares the Spirit coming down upon Jesus gently, the way a dove would. (See: [[rc://en/ta/man/translate/figs-simile]])