fix(docs): fix typo on Building with Shrinkwrap guide

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: If73e00cc2a0ba569829d4c7a76595ef84e85a3be
diff --git a/docs/getting_started/building-with-shrinkwrap.rst b/docs/getting_started/building-with-shrinkwrap.rst
index 6045289..7018e0c 100644
--- a/docs/getting_started/building-with-shrinkwrap.rst
+++ b/docs/getting_started/building-with-shrinkwrap.rst
@@ -93,7 +93,7 @@
 
     .. code-block:: shell
 
-       shrinkwrap run cca-3world.yaml -rtvar=ROOTFS=${SHRINKWRAP_PACKAGE}/cca-3world/rootfs.ext2
+       shrinkwrap run cca-3world.yaml --rtvar=ROOTFS=${SHRINKWRAP_PACKAGE}/cca-3world/rootfs.ext2
 
 Testing RMM with TFTF
 _____________________