README: Mark pen blocks as finished

Author: adazem009

README.md

@@ -146,7 +146,7 @@ int main(int argc, char **argv) {
 - [x] Sprite dragging
 - [x] Touching sprite block
 - [ ] Touching color blocks (color is touching color is not implemented yet)
-- [ ] Pen blocks (all blocks except the stamp block are implemented)
+- [x] Pen blocks
 - [x] Monitors
 - [ ] Graphics effects (color, brightness and ghost are implemented)
 - [x] Speech and thought bubbles