lnwire: fix typo

lnwire/announcement_signatures.go

@@ -5,7 +5,7 @@ import (
 	"io/ioutil"
 )
 
-// AnnounceSignatures this is a direct message between two endpoints of a
+// AnnounceSignatures is a direct message between two endpoints of a
 // channel and serves as an opt-in mechanism to allow the announcement of
 // the channel to the rest of the network. It contains the necessary
 // signatures by the sender to construct the channel announcement message.