Difference in meaning based on position of modal verb
Both are accurate. 梦想成真 is a set phrase, so using 能 before it is acceptable. Note that if it is not a set phrase you need to put 能 before the verb 成真 here, but because it is a set phrase, it is usually perceived/interpreted as a whole. 

You can also break it up and put 能 before 成真. Both are valid here.