Fix a copy-and-paste glitch

pkg/raptorq/interface.go

@@ -141,7 +141,7 @@ type Decoder interface {
  // been freed, calling Encode with its SBN may return an error.
  FreeSourceBlock(sbn uint8)
 
- // Close closes the Decoder. After an Decoder is closed, all methods but
+ // Close closes the Decoder. After a Decoder is closed, all methods but
  // Close() will panic if called.
  Close() error
 }