feat: fix bug in isClosed function where we locked twice 🤦

Signed-off-by: Manan Gupta <manan@planetscale.com>

go/vt/vttablet/tabletmanager/semisyncmonitor/monitor.go

@@ -183,7 +183,7 @@ func (m *Monitor) isSemiSyncBlocked(ctx context.Context) (bool, error) {
 // isClosed returns if the monitor is currently closed or not.
 func (m *Monitor) isClosed() bool {
 	m.mu.Lock()
-	defer m.mu.Lock()
+	defer m.mu.Unlock()
 	return !m.isOpen
 }