docs: Pass relative paths to apibuild.py
    
    Since commit d195cffa2e1b, both $(srcdir) and $(abs_builddir)
    are passed to the apibuild.py script; however, since the
    former is a relative path and the latter an absolute one, the
    script might not be able to detect whether they point to the
    same location.
    
    Pass both as relative paths to avoid the issue.