fix error in Getting Started

[action-hong]

The above template use {{{ meta }}},

and the full demo use:

const context = {
    title: 'vue ssr',
    metas: `
        <meta name="keyword" content="vue,ssr">
        <meta name="description" content="vue srr demo">
    `,
};

it will cause a error： ReferenceError: meta is not defined

change metas to meta can be fixed