Fix 'make docs' target deleting the whole docs subdir
diff --git a/Makefile.am b/Makefile.am
index b9a1a52..4d21b65 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -14,7 +14,7 @@
 	README.md
 
 docs/html: $(top_builddir)/doxygen.cfg $(top_srcdir)/include/plist/*.h
-	rm -rf docs
+	rm -rf docs/html
 	doxygen doxygen.cfg
 
 docs: doxygen.cfg docs/html