How to replace pattern in last line

I'm trying to replace "}," by "}" from the last line of a file : My /etc/firefox/policies/policies.json file like this because I added the certificate lines using a for loop :

$ cat /etc/firefox/policies/policies.json
{
  "policies": {
    "Certificates": {
      "Install": [
        {"cert1.der", "/home/username/cert1.pem"},
        {"cert2.der", "/home/username/cert2.pem"},
$

Please note there no "\n" at the end of this file (yet). Before adding this to the end of my JSON file :

]
    }
  }
}

, how can I remove the last , : replacing "}," by "}" for example ? sed seems a complicated solution to do that according to [this solution](https://stackoverflow.com/a/17115550) . Is there a more readable solution using awk or perl ?