ZipWriter: archive disk numbers are 0 index based

Misunderstood that disk numbers were 1 index based.

Change-Id: I0c1f43436f1c5e9394818ad12164f6d7b982cbd8
diff --git a/libziparchive/zip_writer.cc b/libziparchive/zip_writer.cc
index 21662ec..22a7c53 100644
--- a/libziparchive/zip_writer.cc
+++ b/libziparchive/zip_writer.cc
@@ -368,8 +368,8 @@
 
   EocdRecord er = {};
   er.eocd_signature = EocdRecord::kSignature;
-  er.disk_num = 1;
-  er.cd_start_disk = 1;
+  er.disk_num = 0;
+  er.cd_start_disk = 0;
   er.num_records_on_disk = files_.size();
   er.num_records = files_.size();
   er.cd_size = current_offset_ - startOfCdr;