Use two spaces to indent lists in markdown

We were already doing it in the `docs/` folder, and this is consistent
with the indentations we use in prettly much every programming language.

CONTRIBUTING.md

@@ -10,9 +10,9 @@ The prefered way to report any bug is [opening an issue in the project's repo](h
 
 * Try to use a descriptive and to-the-point title
 * It's a good idea to include some of these sections:
-   * Steps to reproduce the bug
-   * Expected behaviour/response
-   * Actual response
+  * Steps to reproduce the bug
+  * Expected behaviour/response
+  * Actual response
 * Sometimes it is also helpful if you mention your operating system, browser version and installed plugins.
 
 Thanks! :heart: :heart: :heart: