Avoid allocations with (*regexp.Regexp).MatchString

[Juneezee]

What does this do?

We should use (*regexp.Regexp).MatchString instead of (*regexp.Regexp).Match([]byte(...)) when matching string to avoid unnecessary []byte conversions and reduce allocations. A one-line change for free performance improvement.

Example benchmark:

func BenchmarkMatch(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := timeWithSeconds.Match([]byte("06:18:01")); !match {
			b.Fail()
		}
	}
}

func BenchmarkMatchString(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := timeWithSeconds.MatchString("06:18:01"); !match {
			b.Fail()
		}
	}
}

Result:

goos: linux
goarch: amd64
pkg: github.com/go-co-op/gocron
cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkMatch-16          	 7213512	       223.8 ns/op	       8 B/op	       1 allocs/op
BenchmarkMatchString-16    	 8484169	       145.8 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/go-co-op/gocron	4.114s

Which issue(s) does this PR fix/relate to?

List any changes that modify/break current functionality

Have you included tests for your changes?

Did you document any new/modified functionality?

• Updated example_test.go
• Updated README.md

Notes