Change "tägi" to "tagi" in Finnish texts

[etanttila]

Kielitoimiston sanakirja defines tagi more extensively than 'tägi'. The latter just links to 'tagi'. django-colortag uses 'tagi' in Finnish texts, while MOOC-Jutut uses 'tägi'. (Meanwhile a-plus uses less of a loanword "merkintä").
For the sake of consistency and better language usage, we probably should use 'tagi" in MOOC-Jutut as well.

See also discussion in django-colortag PR