Include quotes when we mention a string in the text

[AllenDowney]

There are place where I use monocode font to show the value of a string without including the quotation marks. Luciano suggests we avoid that and I agree.

For example, if we have a line of code like this:

s = 'abc'

And we refer to the string in the text, it should be 'abc', not abc.

On the next pass, let's keep an eye out for them.