Find where you installed it, in my case it was as below because I installed it with Homebrew:
Edit this file:
Remove “.md” from the lines below:
Modula 2 au BufNewFile,BufRead *.m2,*.DEF,*.MOD,*.md,*.mi setf modula2
Search for the following:
au BufNewFile,BufRead *.markdown,*.mdown,*.mkd,*.mkdn,README.md setf markdown
Add the below near the end of line, directly after “README.md”
Restart MacVim and you’ll be good to go.