documentation on making notes may incorrectly suggest CTRL, not ALT

[gitshub]

In "Getting Started" it says to indicate a cell contains Markdown using ctrl+g ctrl+m (g for Gorilla, m for Markdown). But in the help window at upper left of my REPL (using Firefox on Linux Mint) it says most of the commands use ALT, not CTRL, and that is what seems to work, at least for this command.