Merge pull request #4063 from kquick/fix_hash-file_desc

Fixes fall-through to report correct description of hash-file command.

src/nix/hash.cc

@@ -44,6 +44,7 @@ struct CmdHash : Command
         switch (mode) {
         case FileIngestionMethod::Flat:
             d = "print cryptographic hash of a regular file";
+            break;
         case FileIngestionMethod::Recursive:
             d = "print cryptographic hash of the NAR serialisation of a path";
         };