Fix URLs in fenced code blocks being double-parsed

[AugustMiller]

In #464, @leevigraham pointed out that there were some < and > characters sneaking into output—but in way that suggests docblocks are getting parsed "too early" and ignoring fenced code blocks?

This may only be an issue when using the Vuepress-stlyle ::: code components.

No matter, we need to sort out whether this is something we can address with careful formatting in the Craft source, or if there is a solve at the automation level, i.e. with our use of yiisoft/yii2-apidoc (or Vuepress itself).