Adjust contract of create-archive-from-port to match documentation

Trivial change...

The documentation for `create-archive-from-port` says:

    (create-archive-from-port  vault-name                        
                               port                              
                               description                       
                              [#:part-size part-size]) -> string?
      vault-name : string?                                       
      port : input-port?                                         
      description : string?                                      
      part-size : exact-nonnegative-integer? = (* 1024 1024)

This suggests that keyword `#:part-size` is optional, but the original contract requires it to be present.