Rosa—well, that could be anyone! That could be your aunt from South Dakota. Not in this case, though (sorry Aunt Rosa). This poem is talking about the Rosa Parks, the woman who made a major impact on the Civil Rights Movement, not some sweet lady who knits you sweaters for every birthday. While it might seem a little too informal to title the poem Rosa rather than Rosa Parks (you wouldn't address the president as Barack unless you knew him on a personal level), Dove didn't do this to be disrespectful in any way. She did it to humanize this famous figure.
It's easy to look back on historical figures that made big waves in any movement and decide that they were exceptional—more powerful, stronger, more capable of change than we are. What Dove wants to do in this poem, and starting with the familiar-sounding title, is focus on the human, not so much the action.
In focusing on the fact that Rosa Parks was simply an ordinary person named Rosa who did an extraordinary thing, Dove does two things: 1) She actually honors the act more. If Rosa Parks wasn't more powerful, heroic, or in some way super-human, her brave act was that much more admirable. She was probably nervous and scared, but she acted in spite of those feelings. And 2), by reminding us that Parks was just a person like anyone else, she reminds us that we, as readers, are capable of extraordinary acts that could make real social change. Dove accomplishes a lot with a very simple title, much like Rosa Parks accomplished a lot with a very simple act.