Indicate aditional steps - const values: Step 3 - Build your own Match Function

Tutorial code contains comments that point towards additional steps. These do not have a full coverage with the documentation. I was debugging matching function build for some time today. In the end it was purely because one const value was commented out and the compiler was not able to find it.

It would be great to have something indicating to the OpenMatch tutorial users have to uncomment something for the build to work

```go
const (
matchName = "basic-matchfunction"

// Uncomment if following the tutorial
// ticketsPerPoolPerMatch = 4
)
```

Or the tutorial code could have this value integrated into the ```makeMatches()``` function.

```go
func makeMatches(p *pb.MatchProfile, poolTickets map[string][]*pb.Ticket) ([]*pb.Match, error) {
var matches []*pb.Match
count := 0
ticketsPerPoolPerMatch := 4
...
}
```