doc: fix typo in Buffer.from(string, [encoding])

doc: fix typo in Buffer.from(string, [encoding]) · nodejs/node@b10bc31

Original file line number	Diff line number	Diff line change
`@@ -833,7 +833,7 @@ const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');`
`833`	`833`	`console.log(buf2.toString());`
`834`	`834`	```
`835`	`835`
`836`		-A `TypeError` will be thrown if `str` is not a string.
	`836`	+A `TypeError` will be thrown if `string` is not a string.
`837`	`837`
`838`	`838`	`### Class Method: Buffer.isBuffer(obj)`
`839`	`839`	`<!-- YAML`