Fix typo for version 1.18

Signed-off-by: Bruno Costa <bruno.costa@marionete.co.uk>
Signed-off-by: Bruno Costa <ninguex@hotmail.com>

Author: Bruno Costa

content/v1.18/software/upgrade.md

@@ -56,5 +56,5 @@ with the upgrade command.
 
 For example, to maintain the original image registry use 
 ```shell 
-helm upgrade crossplane --namespace crossplane-system crossplane-stable/crossplane --set args={"--registry=index.docker.io"}'
+helm upgrade crossplane --namespace crossplane-system crossplane-stable/crossplane --set 'args={"--registry=index.docker.io"}'
 ```