Use underscore instead of a dash in compat suffixes

The Fedora Packaging Guidelines are clear on this:

> If the base package name ends with a digit, a single underscore (_)
> MUST be appended to the name, and the version MUST be appended to
> that, in order to avoid confusion over where the name ends and the
> version begins.
>
> If the base package name does not end with a digit, the version MUST
> be directly appended to the package name with no intervening
> separator.

Fixes #180.

rust2rpm/__main__.py

@@ -426,7 +426,7 @@ def main():
 
     if args.suffix is not None:
         if metadata.name[-1].isdigit():
-            suffix = f'-{args.suffix}'
+            suffix = f'_{args.suffix}'
         else:
             suffix = args.suffix
     else: