Fix typos, update minimal requirements and Makefile

Created by: LMJ-19

In this pull request I performed the updates suggested in #18 (closed) and #19 (closed).

A small note: the alias open='xdg-open' did not work on my and Martijn's Linux Ubuntu version, since openwas still referring to/bin/openand notxdg-open`. However, we think that users can find the generated html manually as well, so it is not that much of a problem.