Fix binding typo error message[)
Fix binding typo error message[)

Problem: When the signature size is invalid, the error message reports
that "m" has an incorrect size. It should be "sig".

Solution: Replace "m" with "sig" in `binding.c` to resolve the typo.

Fixes: https://github.com/sodium-friends/sodium-native/issues/129

mafintosh/sodium-native,mafintosh/sodium-native,mafintosh/sodium-native,sodium-friends/sodium-native,sodium-friends/sodium-native,mafintosh/sodium-native,sodium-friends/sodium-native