I recently have been annoyed by his own typoes and has been looking for a way to fix it. This is actually pretty simple to cmd-shift-p and install a package called Dictionnaries. Then by hitting F6 you switch on and off the spellcheck highlighting. When you write a lot of documentation that is very helpful (and you want to write a lot of documentation, don’t you ?).

The spellcheck auto-highlight can be enabled for specific file extensions (like .md files) by adding

"spell_check": true

in preferences > Settings - more > Syntax specific - user (while having a markdown page open)