doc: fix typo in writing-test.md · nodejs/node@6f81b27

Original file line number	Diff line number	Diff line change
`@@ -24,7 +24,7 @@ See [directory structure overview][] for outline of existing test and locations.`
`24`	`24`
`25`	`25`	`## How to write a good test`
`26`	`26`
`27`		`-A good test should be written in a style that is optimial for debugging`
	`27`	`+A good test should be written in a style that is optimal for debugging`
`28`	`28`	`when it fails.`
`29`	`29`
`30`	`30`	`In principle, when adding a new test, it should be placed in a new file.`