Fix typo for v5 module path (#13313)

The PR link uses the github repo path, not the module path.

app/web_hub.go

@@ -31,7 +31,7 @@ type WebConnActivityMessage struct {
 
 type Hub struct {
 	// connectionCount should be kept first.
-	// See https://github.com/mattermost/mattermost-server/v5/pull/7281
+	// See https://github.com/mattermost/mattermost-server/pull/7281
 	connectionCount int64
 	app             *App
 	connectionIndex int