README: Mark sound blocks as finished

Author: adazem009

README.md

@@ -124,7 +124,7 @@ libscratchcpp::Project p("https://scratch.mit.edu/projects/XXXXXX");
 
 - [x] Motion blocks
 - [x] Looks blocks
-- [ ] Sound blocks
+- [x] Sound blocks
 - [x] Event blocks
 - [x] Control blocks
 - [ ] Sensing blocks