Ch20: make `:help internal-variables` a link

Link to the official online Vim docs so that readers of the online version don’t have to open up Vim and type the command in.

Any `:help whatever` in the book could be replaced by a link to somewhere in the [online Vim documentation](http://vimdoc.sourceforge.net/htmldoc/), and changing all of them would be even better than changing just this one. But this link in particular is one I wish existed. Whenever I forget the types of scopes, I also forget the name of the Vim help topic about them. Instead, I have to search for [this page](http://learnvimscriptthehardway.stevelosh.com/chapters/20.html) on Google, then run the described command. It be easier if I could just click a link after finding this page. And I’m sure other readers would appreciate the convenience too.

I don’t know how you are publishing this book, but if this is the first link in the whole book, you may need to update your CSS styles or something so that “`:help internal-variables`” doesn’t look blue and underlined in the print version.