[DOC] Fix outdated installation guide (#664)

Author: szha

docs/install.rst

@@ -9,16 +9,16 @@ command installs the latest version of MXNet.
 
 .. code-block:: console
 
-   pip install --upgrade mxnet>=1.3.0
+   pip install --upgrade mxnet>=1.4.0
 
 .. note::
 
    There are other pre-build MXNet packages that enable GPU supports and
-   accelerate CPU performance, please refer to `this tutorial
-   <http://gluon-crash-course.mxnet.io/mxnet_packages.html>`_ for details. Some
+   accelerate CPU performance, please refer to `this page
+   <http://beta.mxnet.io/install.html>`_ for details. Some
    training scripts are recommended to run on GPUs, if you don't have a GPU
-   machine at hands, you may consider `running on AWS
-   <http://gluon-crash-course.mxnet.io/use_aws.html>`_.
+   machine at hand, you may consider `running on AWS
+   <http://d2l.ai/chapter_appendix/aws.html>`_.
 
 
 After installing MXNet, you can install the GluonNLP toolkit by