Small fix to Makefile

I get this error when I do make
pandoc: en/img/bigtree.png: openBinaryFile: does not exist (No such file or directory)
make: *** [epub-en] Error 1

Fixing all image links I get output of epub file (i.e. _img/ -> ../img_), but an error for pdf file
pandoc: Error producing PDF from TeX source.
! Package pdftex.def Error: File `../img/bird.png' not found.

I think the issue is because of pdflatex fails to parse relative path...
The changes in Makefile I think is the most minimal way to fix the build issue..