Fix Makefile by changing README.orafunc to README.asciidoc. Commit
4e0b54de9500ecfe renamed README.orafunc to README.asciidoc but failed to make
necessary changes to the Makefile. This allows "make install" to proceed
without throwing errors

Reported and patch by Beena Emerson, though I decided not to rename the file
back to its original name for now.