doc: add reference to itest in if man-page

Add a cross reference to the itest command.

Fix a typo: %s/Example/Examples/.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/cmd/if.rst b/doc/usage/cmd/if.rst
index 6b3dbe7..813f903 100644
--- a/doc/usage/cmd/if.rst
+++ b/doc/usage/cmd/if.rst
@@ -33,11 +33,11 @@
     $? becomes 0 (true) the statements after the **then** statement will
     be executed. Otherwise the statements after the **else** statement.
 
-Example
--------
+Examples
+--------
 
 The examples shows how the value of a numeric variable can be tested with
-**itest**.
+the :doc:`itest <itest>` command.
 
 ::