PR template: go back to old option name for now

The old one works on all nix(os) versions, whereas the new one doesn't
work e.g. on default 16.03 setting. Discussed on #15112.

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,8 +4,8 @@
 ###### Things done
 
 - [ ] Tested using sandboxing
-  ([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS,
-    or option `build-use-sandbox` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file)
+  ([nix.useChroot](http://nixos.org/nixos/manual/options.html#opt-nix.useChroot) on NixOS,
+    or option `build-use-chroot` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file)
     on non-NixOS)
 - Built on platform(s)
    - [ ] NixOS