tiflash: update tiflash upgrade doc (#2309)
tiflash: update tiflash upgrade doc (#2309)

* tiflash: update tiflash upgrade doc

* test markdownlint

* redo the change

* Apply suggestions from code review

Co-Authored-By: Keke Yi <4b13e11d5beb5669a80650bc9e0535372af5b679@users.noreply.github.com>

Co-authored-by: Keke Yi <4b13e11d5beb5669a80650bc9e0535372af5b679@users.noreply.github.com>
pingcap/docs,pingcap/docs,pingcap/docs