There is a definition of finale in the oed
- The last scene or closing part of a drama or any other public entertainment.
So it's a correct usage of the word, since the world cup is a public entertainment.
I also agree with /u/user8543 that it is used colloquially for any culminating event. Wiktionary says
The grand end of something, especially a show or piece of music.
"Something" is vague enough to include the world cup final, or anything else, although it is rare for it to be used for anything other than a piece of entertainment. For example, you could say
My time at the company culminated in a grand finale when I told my boss "you have no idea how high I can fly".
But as I said, that is quite unusual and sounds a bit unnatural.
The "final" is the specific name of the last match in the world cup. Since it is the culmination of this sporting event, one can say that the final is the finale of the world cup.
The World Cup Final is the finale of the World Cup.
When referring to the match specifically, you would call it "the final".
I wonder whether an England player will score in the final. No, of course they won't, because England will never be in the final.
When referring to the end of the competition in general, you could call it the "finale".
Will the world cup have a spectacular finale, or will it end with a whimper?