[Doc page] Fix launcher page highlighting (#1080)

### Broken highlighting (current)

<img width="800" alt="Screenshot 2023-09-28 at 22 38 15"
src="https://github.com/huggingface/text-generation-inference/assets/11827707/1f07c356-2c3c-4ff0-8ca5-54a032b05d48">

### Fixed highlighting (this PR)

<img width="800" alt="image"
src="https://github.com/huggingface/text-generation-inference/assets/11827707/87fe750d-c26e-4801-95cc-86859a2df52d">

docs/source/basic_tutorials/launcher.md

@@ -2,7 +2,7 @@
 
 <!-- WRAP CODE BLOCKS -->
 
-```
+```shell
 Text Generation Launcher
 
 Usage: text-generation-launcher [OPTIONS]

update_doc.py

@@ -12,7 +12,7 @@ def main():
         "utf-8"
     )
     wrap_code_blocks_flag = "<!-- WRAP CODE BLOCKS -->"
-    final_doc = f"# Text-generation-launcher arguments\n\n{wrap_code_blocks_flag}\n\n```\n{output}\n```"
+    final_doc = f"# Text-generation-launcher arguments\n\n{wrap_code_blocks_flag}\n\n```shell\n{output}\n```"
 
     filename = "docs/source/basic_tutorials/launcher.md"
     if args.check: