dev/how-to/migrate: update from-mysql (#1396)
dev/how-to/migrate: update from-mysql (#1396)

* dev/how-to/migrate: update from-mysql

* Update dev/how-to/migrate/from-mysql.md

Co-Authored-By: Lilian Lee <c684465c17314189c18707959627e2ec4fdec2d0@pingcap.com>

* Update dev/how-to/migrate/from-mysql.md

Co-Authored-By: Lilian Lee <c684465c17314189c18707959627e2ec4fdec2d0@pingcap.com>

* Address a comment

Delete the unnecessary code block format.

pingcap/docs-cn,pingcap/docs-cn