PULL_REQUEST_TEMPLATE.md: ticks to bullets

As suggested by 06kellyjac.

.github/PULL_REQUEST_TEMPLATE.md

@@ -22,10 +22,10 @@ Reviewing guidelines: https://nixos.org/manual/nixpkgs/unstable/#chap-reviewing-
   - [ ] aarch64-darwin
 - [ ] For non-Linux: Is `sandbox = true` set in `nix.conf`? (See [Nix manual](https://nixos.org/manual/nix/stable/#sec-conf-file))
 - [ ] Tested, as applicable:
-  - [ ] [NixOS test(s)](https://nixos.org/manual/nixos/unstable/index.html#sec-nixos-tests) (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
-  - [ ] and/or [package tests](https://nixos.org/manual/nixpkgs/unstable/#sec-package-tests)
-  - [ ] or, for functions and "core" functionality, tests in [lib/tests](https://github.com/NixOS/nixpkgs/blob/master/lib/tests) or [pkgs/test](https://github.com/NixOS/nixpkgs/blob/master/pkgs/test)
-  - [ ] made sure NixOS tests are [linked](https://nixos.org/manual/nixpkgs/unstable/#ssec-nixos-tests-linking) to the relevant packages
+  - [NixOS test(s)](https://nixos.org/manual/nixos/unstable/index.html#sec-nixos-tests) (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
+  - and/or [package tests](https://nixos.org/manual/nixpkgs/unstable/#sec-package-tests)
+  - or, for functions and "core" functionality, tests in [lib/tests](https://github.com/NixOS/nixpkgs/blob/master/lib/tests) or [pkgs/test](https://github.com/NixOS/nixpkgs/blob/master/pkgs/test)
+  - made sure NixOS tests are [linked](https://nixos.org/manual/nixpkgs/unstable/#ssec-nixos-tests-linking) to the relevant packages
 - [ ] Tested compilation of all packages that depend on this change using `nix-shell -p nixpkgs-review --run "nixpkgs-review wip"`
 - [ ] Tested basic functionality of all binary files (usually in `./result/bin/`)
 - [21.11 Release Notes (or backporting 21.05 Release notes)](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#generating-2111-release-notes)