[BPK-XXX] Fix typo in web button readme

`-alt` icons were deprecated see https://github.com/Skyscanner/backpack/blob/master/changelog.md#2017-01-13---deprecated-stubby-arrow-icon-and-fixed-onclose-firing-twice-bug-in-the-modal

Author: Matthew Davidson

packages/bpk-component-button/readme.md

@@ -13,7 +13,7 @@ npm install bpk-component-button --save-dev
 ```js
 import React from 'react';
 import { withButtonAlignment, withRtlSupport } from 'bpk-component-icon';
-import ArrowIcon from 'bpk-component-icon/sm/long-arrow-right-alt';
+import ArrowIcon from 'bpk-component-icon/sm/long-arrow-right';
 import BpkButton from 'bpk-component-button';
 
 const AlignedArrowIcon = withButtonAlignment(withRtlSupport(ArrowIcon));