Use hosts in acl documentation

thx @efficacy38 for noticing this

Ref: juanfont#1863

docs/ref/acls.md

@@ -87,7 +87,7 @@ Here are the ACL's to implement the same permissions as above:
   // to define a single host, use a /32 mask. You cannot use DNS entries here,
   // as they're prone to be hijacked by replacing their IP addresses.
   // see https://github.com/tailscale/tailscale/issues/3800 for more information.
-  "Hosts": {
+  "hosts": {
     "postgresql.internal": "10.20.0.2/32",
     "webservers.internal": "10.20.10.1/29"
   },