change pip command since pypi is available (#306)

Author: duyifanict

docs/getting_started.md

@@ -28,7 +28,7 @@ Now, let's get started on exploring how to realize your time series analysis pro
 We recommend installing BasicTS on a Linux system (such as Ubuntu or CentOS) with Python 3.8 or higher:
 
 ```bash
-pip install basicts-xxx-py3-x.whl
+pip install basicts
 ```
 
 We recommend using https://docs.conda.io/en/latest/miniconda.html or https://www.anaconda.com/ to create a virtual Python environment.

docs/getting_started_cn.md

@@ -28,7 +28,7 @@
 我们建议在 Linux 系统（如 Ubuntu 或 CentOS）上，在 Python 3.8 或更高版本上安装 BasicTS：
 
 ```bash
-pip install basicts-xxx-py3-x.whl
+pip install basicts
 ```
 
 我们推荐使用 [Miniconda](https://docs.conda.io/en/latest/miniconda.html) 或 [Anaconda](https://www.anaconda.com/) 来创建虚拟 Python 环境。