Fix file-based persistence for challenges. (#14)

The `AddFluffySpoonLetsEncryptFileChallengePersistence()` extension accidentally stored the challenge in the cert file, rather than a challenge file. 

Fixes #7

src/FluffySpoon.AspNet.LetsEncrypt/RegistrationExtensions.cs

@@ -92,7 +92,7 @@ public static void AddFluffySpoonLetsEncryptFileChallengePersistence(
 		  this IServiceCollection services,
 		  string relativeFilePath = "FluffySpoonAspNetLetsEncryptChallenge")
 		{
-			AddFluffySpoonLetsEncryptCertificatePersistence(services,
+			AddFluffySpoonLetsEncryptChallengePersistence(services,
 				new FilePersistenceStrategy(relativeFilePath));
 		}