I have figured it out.
Find /vendor/flarum/core/locale/core.yml
Search with_page_title
, you will find with_page_title: "{pageNumber, plural, =1 {{pageTitle} - {forumName}} other {{pageTitle}: Page # - {forumName}}}"
Change this line into
with_page_title: "{pageNumber, plural, =1 {{pageTitle}} other {Page # - {pageTitle}}}"
Clear the cache, all done.