docs(source,go-bindata) update go-bindata source readme to include regex overrides for asset names

[erindatkinson]

When trying to run on a preconfigured bindata package I had, I ran into the issue that the migrations were not being run when done with code, even when they were valid with the cli runner. This was silently failing due to the continue in the WithInstance call failing on no regex match for otherwise valid migrations.

This change updates the docs to explain better how to enable directoried asset names in lieu of just single filenames. Rather than try to update the default regex, adding the documentation on how to override to your own specific needs seems like a better option.

The example is based off the existing one, but updated based on my local tests, which worked upon compile/running:

Expand: database.go

package database

import (
	"log/slog"
	"regexp"

	"github.com/erindatkinson/go-testing/internal/database/migrations"

	"github.com/golang-migrate/migrate/v4"
	"github.com/golang-migrate/migrate/v4/source"
	bindata "github.com/golang-migrate/migrate/v4/source/go_bindata"
)

func Test() {
	source.Regex = regexp.MustCompile(`^[a-zA-Z\-_\/]*([0-9]+)_(.*)\.(down|up)\.(.*)$`)
	s := bindata.Resource(migrations.AssetNames(),
		func(name string) ([]byte, error) {
			return migrations.Asset(name)
		})

	instance, err := bindata.WithInstance(s)
	if err != nil {
		slog.Error("source", "error", err)
		return
	}

	m, err := migrate.NewWithSourceInstance("sqlite3", instance, "sqlite3://data/test.sqlite3")
	if err != nil {
		slog.Error("mDriver", "error", err)
		return
	}

	if err = m.Up(); err != nil {
		slog.Error("up", "error", err)
	}
}