Convert registering-users.md to use ``` syntax for code blocks

The bit about the matrix-make-user-admin script was messed up (it wasn't actually a code block so the "<username>" was hidden). For me at least it seems like the ``` syntax is much harder to accidentally mess up.

docs/registering-users.md

@@ -4,11 +4,15 @@ Run this to create a new user account on your Matrix server.
 
 You can do it via this Ansible playbook (make sure to edit the `<your-username>` and `<your-password>` part below):
 
-	ansible-playbook -i inventory/hosts setup.yml --extra-vars='username=<your-username> password=<your-password> admin=<yes|no>' --tags=register-user
+```
+ansible-playbook -i inventory/hosts setup.yml --extra-vars='username=<your-username> password=<your-password> admin=<yes|no>' --tags=register-user
+```
 
 **or** using the command-line after **SSH**-ing to your server (requires that [all services have been started](#starting-the-services)):
 
-	/usr/local/bin/matrix-synapse-register-user <your-username> <your-password> <admin access: 0 or 1>
+```
+/usr/local/bin/matrix-synapse-register-user <your-username> <your-password> <admin access: 0 or 1>
+```
 
 **Note**: `<your-username>` is just a plain username (like `john`), not your full `@<username>:<your-domain>` identifier.
 
@@ -25,5 +29,6 @@ The script `/usr/local/bin/matrix-make-user-admin` may be used to upgrade a user
 * log on to your server with ssh
 * execute with the username:
 
-    /usr/local/bin/matrix-make-user-admin <username>
-
+```
+/usr/local/bin/matrix-make-user-admin <username>
+```