Reformat commands in contributor guide

The commands were prefixed by "#", evidently to indicate a command prompt. But that is the shell comment syntax, so it
is very unintuitive.

Author: per1234

.github/CONTRIBUTING.md

@@ -2,14 +2,14 @@
 
 To work on the codebase you have to install all the dependencies:
 
-```sh
-# npm install
+```
+npm install
 ```
 
 To run tests set the environment variable `GITHUB_TOKEN` with a valid Personal Access Token and then:
 
-```sh
-# npm run test
+```
+npm run test
 ```
 
 See the [official Github documentation][pat-docs] to learn more about Personal Access Tokens.