Add missing install command for brew on macOS

INSTALL.md

@@ -25,7 +25,7 @@ These commands install [Homebrew](https://brew.sh), which simplifies installing
  
 To upgrade to a newer version, simply run `pip3 install -U katrain`
 
-If you encounter an error about SDL being missing, try ` brew sdl sdl_image sdl_mixer sdl_ttf portmidi`
+If you encounter an error about SDL being missing, try ` brew install sdl sdl_image sdl_mixer sdl_ttf portmidi`
 
 ### <a name="MacSources"></a>Troubleshooting and Installation from sources