Specify shell language in code snippets

Fix code samples displayed in gogs markdown renderer.

README.md

@@ -7,12 +7,12 @@ A remake of pong up to 4 players
 You need `make` and `SDL1` to build this project.
 
 Example on Ubuntu :
-```
+```sh
 # apt install make g++ libsdl-dev
 $ make
 ```
 
 To execute :
-```
+```sh
 $ make exec
 ```