Properly render ACL in documentation

The ACL example in the documentation specifies the highlighting as JSON, but it has comments which causes GitHub to render them as errors. Comments are allowed in the JSON5 extension, however, to which this patch switches.

docs/acls.md

@@ -57,7 +57,7 @@ To use ACLs in headscale, you must edit your config.yaml file. In there you will
 
 Here are the ACL's to implement the same permissions as above:
 
-```json
+```json5
 {
   // groups are collections of users having a common scope. A user can be in multiple groups
   // groups cannot be composed of groups