fix use of x-* attribute in go templates (#22403)

## Description

fix
https://docs.docker.com/compose/bridge/customize/#add-your-own-templates
as attributes with a `-` can't be accessed directly using dot-notation
in a go template. See
https://stackoverflow.com/questions/48146448/range-through-values-within-variable-containing-dashes-in-golang-templates

## Related issues or tickets

## Reviews

- [ ] Technical review
- [ ] Editorial review
- [ ] Product review

Signed-off-by: Nicolas De Loof <[email protected]>

Author: ndeloof

content/manuals/compose/bridge/customize.md

@@ -133,8 +133,8 @@ metadata:
 spec:
   rules:  
 {{ range $name, $service := .services }}
-{{ if $service.x-virtual-host }}
-  - host: ${{ $service.x-virtual-host }}
+{{ range index $service "x-virtual-host" }}
+  - host: ${{ . }}
     http:
       paths:
       - path: "/"