Improve _PHP: Hypertext Preprocessor_ presentation

[timacdonald]

As this text is prose and not computer output, I would argue that literal should not be used and, if anything, we should use emphasis for visual reasons.

We are also inconsistently quoting the initialism expansion, i.e., PHP: Hypertext Preprocessor. Removing the quotes makes sense to me because we will present the term in italics.

I've also added the missing closing comma for the Preface parenthetical.

Discussion point: I don't love the What is PHP? parenthetical. It feels clunky. I would propose that we duplicate the Preface parenthetical.

PHP (recursive acronym for PHP: Hypertext Preprocessor), which stands for PHP: Hypertext Preprocessor, is a widely-used open source general-purpose scripting language that is especially suited for web development and can be embedded into HTML.

I didn't make this change in this PR as it felt more opinionated. Love to hear your thoughts.

Before

After