fix: update code comments for WithNumCompactors

[rfyiamcool]

😁 I think the default value of NumCompactors is 4 in comments for WithNumCompactors, it's not 2.

Because the value of NumCompactors is 4 in DefaultOptions function.

https://github.com/dgraph-io/badger/blob/main/options.go#L148

DefaultOptions

// DefaultOptions sets a list of recommended options for good performance.
// Feel free to modify these to suit your needs with the WithX methods.
func DefaultOptions(path string) Options {
	return Options{
		// ...

		NumCompactors:           4, // Run at least 2 compactors. Zero-th compactor prioritizes L0.

		// ...
}

code comments for WithNumCompactors.

// WithNumCompactors sets the number of compaction workers to run concurrently.  Setting this to
// zero stops compactions, which could eventually cause writes to block forever.
//
// The default value of NumCompactors is 2. One is dedicated just for L0 and L1.
func (opt Options) WithNumCompactors(val int) Options {
	opt.NumCompactors = val
	return opt
}

[skrdgraph]

Thanks @rfyiamcool for helping out here.