When trying to determine the rhyme pattern in a poem or just a part of it, you only have to look at the final words in each line - based on which words rhyme, you will know what the rhyme pattern is.
In this stanza, we have the following pattern: AABB. This means that song (the last word in the first line) rhymes with wrong (the last word in the second line) which is why they are both A. B refers to the words span and man (the last words in lines 3 and 4) which also rhyme.
