Fix English grammar

follow
611c981346 (commitcomment-20407445)

site/theme/en-US.js

@@ -14,7 +14,7 @@ module.exports = {
     'app.content.edit-page': 'Edit this page on Github!',
     'app.content.edit-demo': 'Edit this demo on Github!',
     'app.component.examples': 'Examples',
-    'app.home.slogan': 'An UI Design Language',
+    'app.home.slogan': 'A UI Design Language',
     'app.home.introduce': 'Introduce',
     'app.home.start': 'Get Started',
     'app.home.best-practice': 'Best Practice',

site/theme/static/template.html

@@ -7,7 +7,7 @@
     <meta name="description" content="An enterprise-class UI design language and React-based implementation">
     <meta name="description" content="A set of high-quality React components">
     <meta name="description" content="One of best React UI library for enterprise background">
-    <title>Ant Design - An UI Design Language</title>
+    <title>Ant Design - A UI Design Language</title>
     <link rel="icon" href="https://t.alipayobjects.com/images/T1QUBfXo4fXXXXXXXX.png" type="image/x-icon">
     <link rel="stylesheet" type="text/css" href="https://fonts.googleapis.cnpmjs.org/css?family=Lato:400,700|Raleway"/>
     <link rel="stylesheet" type="text/css" href="{{ root }}index-1.css"/>