fix #658: update symbol font URL
fix #658: update symbol font URL

after github.com/geometalab/osmaxx-symbology has been split from github.com/geometalab/osmaxx-docs

geometalab/osmaxx-frontend,geometalab/osmaxx,geometalab/osmaxx-frontend,geometalab/osmaxx,geometalab/osmaxx-frontend,geometalab/osmaxx,geometalab/osmaxx,geometalab/osmaxx-frontend