nixos/release-notes: move "PHP config-file-scan-dir" from 16.09 to 17.03

Commits

  351d1243 ("nixos/release-notes: PHP config-file-scan-dir /etc -> /etc/php.d")
  41c8aa8d ("php: change config-file-scan-dir from /etc to /etc/php.d")

were merged to master _after_ NixOS 16.09. Commit 351d1243 then wrongly
updated the NixSO 16.09 release notes. Fix by moving the entry to NixOS
17.03.