fix typo in shntool README

Hi, there is a typo in shntool README: wavepack should be wavpack

Index: pkg/README
===================================================================
RCS file: /cvs/ports/audio/shntool/pkg/README,v
retrieving revision 1.2
diff -u -p -r1.2 README
--- pkg/README 4 Sep 2018 12:46:09 -0000 1.2
+++ pkg/README 28 Sep 2018 14:33:29 -0000
@@ -14,5 +14,5 @@ To support these extra formats, install
- shorten (.shn)
- flac (.flac)
- mac (.ape)
-- wavepack (.wv)
+- wavpack (.wv)
- bonk (.bonk)