use correct code block markers

Author: tshepang

src/conventions.md

@@ -34,7 +34,7 @@ The compiler contains some C++ code for interfacing with parts of LLVM that
 don't have a stable C API.
 When modifying that code, use this command to format it:
 
-```sh
+```console
 ./x test tidy --extra-checks=cpp:fmt --bless
 ```
 
@@ -47,12 +47,12 @@ The Rust repository contains quite a lot of Python code. We try to keep
 it both linted and formatted by the [ruff][ruff] tool.
 
 When modifying Python code, use this command to format it:
-```sh
+```console
 ./x test tidy --extra-checks=py:fmt --bless
 ```
 
 and the following command to run lints:
-```sh
+```console
 ./x test tidy --extra-checks=py:lint
 ```