discount: 2.2.2 -> 2.2.3

Semi-automatic update. These checks were done:

- built on NixOS
- ran `/nix/store/rq0ny7n6qfg7sgjqa5ki2ds6v3gzjwil-discount-2.2.3/bin/markdown help` got 0 exit code
- ran `/nix/store/rq0ny7n6qfg7sgjqa5ki2ds6v3gzjwil-discount-2.2.3/bin/markdown -V` and found version 2.2.3
- ran `/nix/store/rq0ny7n6qfg7sgjqa5ki2ds6v3gzjwil-discount-2.2.3/bin/markdown --version` and found version 2.2.3
- found 2.2.3 with grep in /nix/store/rq0ny7n6qfg7sgjqa5ki2ds6v3gzjwil-discount-2.2.3
- found 2.2.3 in filename of file in /nix/store/rq0ny7n6qfg7sgjqa5ki2ds6v3gzjwil-discount-2.2.3