When I google the lyrics I get "spring's a girl" sometimes and "springs a girl" other times. My instincts tell me that the apostrophe is more likely the correct version. I won't go into the semantics of a grammatical discussion unless you force me to ;-) Further, my instincts tell me that, when he mentions the girl in the streets, he's still talking about his "love" mentioned in the first verse. Even further, as he's already mentioned spring in an earlier verse, my instincts are telling me that he's equating his girl with the delights of the coming spring. And I'd like to bet that, even if you could ask him, he wouldn't tell you!