sync: Add monitor tests

sync/monitor.go

@@ -25,7 +25,7 @@ func (this *Monitor[T]) Set (value T) {
 // Borrow borrows the value from the Monitor, and returns a function that must
 // immediately be deferred, like this:
 //
-//   value, done := Monitor.Borrow()
+//   value, done := monitor.Borrow()
 //   defer done()
 //
 // From the time Borrow is called to the time the done function is called, it is