docs: Correct typo 'simlink' -> 'symlink'
docs: Correct typo 'simlink' -> 'symlink'

$ flit install --help |& grep -e '-s'
usage: flit install [-h] [-s] [--pth-file] [--user] [--env] [--python PYTHON]
  -s, --symlink         Symlink the module/package into site packages instead

poliastro/poliastro