Fix typo on form's index.en-US.md (#165)

Author: lubien

components/form/index.en-US.md

@@ -150,7 +150,7 @@ After wrapped by `getFieldDecorator`, `value`(or other property defined by `valu
 | -------- | ----------- | ---- | ------------- |
 | id | The unique identifier is required. support [nested fields format](https://github.com/react-component/form/pull/48). | string |  |
 | options.getValueFromEvent | Specify how to get value from event or other onChange arguments | function(..args) | [reference](https://github.com/react-component/form#option-object) |
-| options.initialValue | You can specify initial value, type, optional value of children node. (Note: Because `Form` will test equality with `===` internaly, we recommend to use vairable as `initialValue`, instead of literal) |  | n/a |
+| options.initialValue | You can specify initial value, type, optional value of children node. (Note: Because `Form` will test equality with `===` internaly, we recommend to use variable as `initialValue`, instead of literal) |  | n/a |
 | options.normalize | Normalize value to form component, [a select-all example](https://codesandbox.io/s/kw4l2vqqmv) | function(value, prevValue, allValues): any | - |
 | options.rules | Includes validation rules. Please refer to "Validation Rules" part for details. | object\[] | n/a |
 | options.trigger | When to collect the value of children node | string | 'change' |