docs: remove tag 'next' from installation instructions (#1686)

It seems to point to an older version (1.0.0-rc.1 instead of 1.0.2).

Author: mulmschneider

packages/docs/docs/README.md

@@ -29,8 +29,8 @@ footer: MIT Licensed | Copyright © 2018-present Evan You
 
 ``` bash
 # install
-yarn global add vuepress@next 
-# OR npm install -g vuepress@next
+yarn global add vuepress
+# OR npm install -g vuepress
 
 # create a markdown file
 echo '# Hello VuePress' > README.md

packages/docs/docs/guide/getting-started.md

@@ -10,7 +10,7 @@ If you just want to play around with VuePress, you can install it globally:
 
 ``` bash
 # install globally
-yarn global add vuepress@next # OR npm install -g vuepress@next
+yarn global add vuepress # OR npm install -g vuepress
 
 # create a markdown file
 echo '# Hello VuePress' > README.md
@@ -28,7 +28,7 @@ If you have an existing project and would like to keep documentation inside the
 
 ``` bash
 # install as a local dependency
-yarn add -D vuepress@next # OR npm install -D vuepress@next
+yarn add -D vuepress # OR npm install -D vuepress
 
 # create a docs directory
 mkdir docs

packages/docs/docs/zh/guide/getting-started.md

@@ -10,7 +10,7 @@
 
 ``` bash
 # 安装
-yarn global add vuepress@next # 或者：npm install -g vuepress@next
+yarn global add vuepress # 或者：npm install -g vuepress
 
 # 新建一个 markdown 文件
 echo '# Hello VuePress!' > README.md
@@ -28,7 +28,7 @@ vuepress build .
 
 ``` bash
 # 将 VuePress 作为一个本地依赖安装
-yarn add -D vuepress@next # 或者：npm install -D vuepress@next
+yarn add -D vuepress # 或者：npm install -D vuepress
 
 # 新建一个 docs 文件夹
 mkdir docs