Reformat all code to use shell instead of sh

snippets/add-submodule.md

@@ -7,11 +7,11 @@ Adds a new submodule to the repository.
 
 - Use `git submodule add <upstream-path> <local-path>` to add a new submodule from `<upstream-path>` to `<local-path>`.
 
-```sh
+```shell
 git submodule add <upstream-path> <local-path>
 ```
 
-```sh
+```shell
 git submodule add https://github.com/30-seconds/30-seconds-of-code ./30code
 # Creates the directory `30code` containing the submodule from
 # "https://github.com/30-seconds/30-seconds-of-code"