Ensure that iTFRewritePackageOrder is "MD5sum" to match apt-ftparchive

The iTFRewritePackageOrder is used in indexcopy to copy and normalize
cdrom Packages files. This change will ensure that there is no
"normalization" that changes MD5sum -> MD5Sum which alters the hash
of the Packages file on disk (oh the irony).

apt-pkg/tagfile.cc

@@ -526,7 +526,7 @@ static const char *iTFRewritePackageOrder[] = {
                           "Conffiles",
                           "Filename",
                           "Size",
-                          "MD5Sum",
+                          "MD5sum",
                           "SHA1",
                           "SHA256",
                           "SHA512",