all: Fix comment typos

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GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4481

lib/protocol/protocol.go

@@ -127,7 +127,7 @@ type message interface {
 
 type asyncMessage struct {
 	msg  message
-	done chan struct{} // done closes when we're done marshalling the message and it's contents can be reused
+	done chan struct{} // done closes when we're done marshalling the message and its contents can be reused
 }
 
 const (