lint: specify language for fenced code blocks

Author: elliotwutingfeng

README.md

@@ -15,21 +15,21 @@ Linux only for now.
 
 ### Cargo
 
-```
+```shell
 cargo install writedisk
 ```
 
 ### Nix/NixOS
 
 Per user:
 
-```
+```shell
 nix-env --install writedisk
 ```
 
 System-wide:
 
-```
+```shell
 environment.systemPackages = with pkgs; [ writedisk ];
 ```