README: Add missing code block

One of the command sequences was not indented enough, so it appears as
text instead of code.

This patch fixes that by surrounding it with ``` to mark it explicitly
as a code block.

README.md

@@ -151,9 +151,10 @@ To use them, they just need to be available in your system.
 
 For example, in Debian you can run the following to install all three:
 
+```
 apt install greylistd spamc clamdscan
 usermod -a -G greylist mail
-
+```
 
 Note that the default hook may not work in all cases, it is provided as a
 practical example but you should adjust it to your particular system if