README: Mark up one-line code blocks by four spaces instead of using ```

Using four spaces to indent code is also the [genuine Markdown
syntax][1].

[1]: https://daringfireball.net/projects/markdown/syntax#precode

README.md

@@ -238,23 +238,17 @@ on all the following open-source repositories:
   - GitHub
     https://github.com/SoftEtherVPN/SoftEtherVPN/
 
-    ```
-    $ git clone https://github.com/SoftEtherVPN/SoftEtherVPN.git
-    ```
+        $ git clone https://github.com/SoftEtherVPN/SoftEtherVPN.git
 
   - GitLab (mirrored from GitHub)
     https://gitlab.com/SoftEther/SoftEtherVPN/
 
-    ```
-    $ git clone https://gitlab.com/SoftEther/SoftEtherVPN.git
-    ```
+        $ git clone https://gitlab.com/SoftEther/SoftEtherVPN.git
 
   - Codeberg (mirrored from GitHub)
     https://codeberg.org/softether/vpn
 
-    ```
-    $ git clone https://codeberg.org/softether/vpn.git
-    ```
+        $ git clone https://codeberg.org/softether/vpn.git
 
 We hope that you can reach one of the above URLs at least!