In order for Grunt to recursively watch for modifications of any HTML files inside of app/views (particularly the new templates/watchlist-panel.html file), we need to change line 59 of our Gruntfile.js.
The text in the book says to change it to the following:
However, that string contains 3 single quotation marks, which subsequently leads to a syntax error message from Grunt:
(The ^ points to the dot right before 'html'.)

Any idea what the correct syntax of this regex should be?

Found the solution myself:
Now Grunt watches all the subfolders as well.