Fix typo code of isNumber

```js
// BEFORE
var isNumber = require('101/is-number');

['foo', 'bar', 1].map(isString); // [false, false, true]
```

```js
// AFTER
var isNumber = require('101/is-number');

['foo', 'bar', 1].map(isNumber); // [false, false, true]
```