Fix MXNet installation error using pip. modified:   chapter_installation/index.md
Fix MXNet installation error using pip.
modified:   chapter_installation/index.md

mli/gluon-tutorials-zh,d2l-ai/d2l-zh,mli/gluon-tutorials-zh,d2l-ai/d2l-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh,d2l-ai/d2l-zh