community/sphinx: replace missing snowball-dev with libstemmer-dev

https://build.alpinelinux.org/buildlogs/build-3-15-armv7/community/sphinx/sphinx-2.2.11-r4.log

ERROR: unable to select packages:
  snowball-dev (no such package):
    required by: .makedepends-sphinx-20211020.024427[snowball-dev]