updated brew command

[falkorichter]

brew cask install qlmarkdown

Error: brew caskis no longer abrewcommand. Usebrew --cask instead.

[MaykThewessen]

brew install qlmarkdown

Also worked, installed v1.5.0 (51)

[gutogarrote]

brew install qlmarkdown

Also worked, installed v1.5.0 (51)

brew install qlmarkdown doesn't install from this repo (https://github.com/toland/qlmarkdown), it installs from another project https://github.com/sbarex/QLMarkdown.
So you installed a different app (that does the same as this, but also does other things).
This toland/qlmarkdown repo here copies QLMarkdown.qlgenerator to the folder ~/Library/QuickLook
The other repo is a newer, more complex app.