Fix typed logging

Looks like I broke this when I added Sorbet types. I don't think this
affected any released gems, but my logs were filling up with:

```
    ____  _                        ___ _____          __
   / __ \(_)________ ___  ______ _/ (_) __(_)__  ____/ /
  / / / / / ___/ __ `/ / / / __ `/ / / /_/ / _ \/ __  /
 / /_/ / (__  ) /_/ / /_/ / /_/ / / / __/ /  __/ /_/ /
/_____/_/____/\__, /\__,_/\__,_/_/_/_/ /_/\___/\__,_/
                /_/                          v0.5.0
{ delay: 1.0..5.0, pool_size: 5, error_hooks_size: 1 }
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
T::Private::Types::Void::VOID
```

lib/disqualified/logging.rb

@@ -13,12 +13,12 @@ module Disqualified::Logging
 
   module_function
 
-  sig { params(parts: T.untyped).void }
+  sig { params(parts: T.untyped).returns(String) }
   def format_log(*parts)
     *extras, message = parts
 
     if extras.empty?
-      message
+      message.to_s
     else
       "#{extras.map { |x| "[#{x}]" }.join(" ")} #{message}"
     end