doc: missing :: in doc/usage/cmd/itest.rst

Add :: for correct formatting of example.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/cmd/itest.rst b/doc/usage/cmd/itest.rst
index ab933db..9c307fb 100644
--- a/doc/usage/cmd/itest.rst
+++ b/doc/usage/cmd/itest.rst
@@ -86,6 +86,8 @@
 Postfix .s causes a string compare. The string '0xa1234' is alphabetically
 smaller than '0xb'.
 
+::
+
     => if itest.s 0xa1234 < 0xb; then echo true; else echo false; fi
     true