public-acme: the roots field is named roots, not root

step-ca (the other acme-ca provider) already spells it correctly; route-
proxy's own template reads ${bound:acme-ca:roots}. Found live, assigning
public-acme for the first time tonight: the mesh refused the push outright
rather than composing a broken binding — 'route-proxy asks its acme-ca
binding for roots, and what answers it says ... root'. Empty stays empty:
a public CA's root is the system trust store already, per route-proxy's
own design (an empty ACME_CA_BUNDLE means exactly that).

modules/public-acme/module.json

@@ -13,7 +13,7 @@
       "at": "acme-v02.api.letsencrypt.org",
       "port": 443,
       "path": "/directory",
-      "root": ""
+      "roots": ""
     }
   }
 }