fix typo in preface.md (#366)
fix typo in preface.md (#366)

* fix typo in preface.md

* update wording in preface.md

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