fix link in build.md
fix link in build.md

close #1868
RemiKoutcherawy/exist,hungerburg/exist,lcahlander/exist,wolfgangmm/exist,windauer/exist,RemiKoutcherawy/exist,olvidalo/exist,windauer/exist,ambs/exist,ambs/exist,windauer/exist,opax/exist,ljo/exist,eXist-db/exist,ljo/exist,eXist-db/exist,dizzzz/exist,olvidalo/exist,wolfgangmm/exist,adamretter/exist,dizzzz/exist,adamretter/exist,ljo/exist,eXist-db/exist,RemiKoutcherawy/exist,dizzzz/exist,windauer/exist,ambs/exist,hungerburg/exist,lcahlander/exist,RemiKoutcherawy/exist,hungerburg/exist,dizzzz/exist,eXist-db/exist,ljo/exist,adamretter/exist,opax/exist,wolfgangmm/exist,eXist-db/exist,opax/exist,ambs/exist,windauer/exist,lcahlander/exist,wolfgangmm/exist,olvidalo/exist,olvidalo/exist,adamretter/exist,opax/exist,wolfgangmm/exist,ljo/exist,lcahlander/exist,adamretter/exist,windauer/exist,ambs/exist,wolfgangmm/exist,olvidalo/exist,eXist-db/exist,lcahlander/exist,ambs/exist,opax/exist,RemiKoutcherawy/exist,RemiKoutcherawy/exist,hungerburg/exist,dizzzz/exist,adamretter/exist,dizzzz/exist,lcahlander/exist,hungerburg/exist,ljo/exist