Page 41 fourth line from bottom asked that I open the
earlier saved markup in my Text Editor.
Open your Text Editor the same way you did when you first started writing markup. Not the browser. If your program defaults to a "New File" then it should be blank, waiting for you to type in it. Instead, go open your old index.html file (probably File-> Open -> navigate to where you saved that file) from the Text Editor itself.
I don't know if Macs are like Linux in this way but I assume this is universal: you can also find the index.html in the folder you're keeping it in, right click (whatever the Mac version of right-click is) and choose "open in..." where you can say Open with my Text Editor, rather than the default, which seems to be Open In My Browser.
The browser will never show markup unless you somehow saved the file as a text file (with the MimeType being text/plain rather than text/html) however most systems see that .html at the end and assign the correct (text/html) mime type so that browsers show the page you want rather than the markup.