Fix deb install instructions: ${VERSION}_foo (#5542)

The install instructions weren't copy-pastable: `$VERSION_foo` is the same as `${VERSION_foo}`, not `${VERSION}_foo`.

```sh
$ VERSION=4.6.1
$ curl -fOL https://github.com/coder/code-server/releases/download/v$VERSION/code-server_$VERSION_amd64.deb
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     9    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (22) The requested URL returned error: 404
$ echo curl -fOL https://github.com/coder/code-server/releases/download/v$VERSION/code-server_$VERSION_amd64.deb
curl -fOL https://github.com/coder/code-server/releases/download/v4.6.1/code-server_.deb
```

Author: ssbr

docs/install.md

@@ -154,8 +154,8 @@ code-server
 > upgrade or [build with npm](#npm).
 
 ```bash
-curl -fOL https://github.com/coder/code-server/releases/download/v$VERSION/code-server_$VERSION_amd64.deb
-sudo dpkg -i code-server_$VERSION_amd64.deb
+curl -fOL https://github.com/coder/code-server/releases/download/v$VERSION/code-server_${VERSION}_amd64.deb
+sudo dpkg -i code-server_${VERSION}_amd64.deb
 sudo systemctl enable --now code-server@$USER
 # Now visit http://127.0.0.1:8080. Your password is in ~/.config/code-server/config.yaml
 ```