fix: typo in surprise command

README document had a typo in the surprise command where it was mentioned as `suprise` instead of `surprise`.
Which was resulting in *repository does not exist* error.

README.md

@@ -6,7 +6,7 @@ Happy Birthday, Docker!
 March 2019 marks Docker's sixth year, and we thought we'd celebrate it docker-style with a birthday surprise!
 To try it out, run:
 
-`docker run -it --rm suprise`
+`docker run -it --rm surprise`
 
 Complete source can be found [here](https://github.com/docker/birthday).