fix(docsApp): show correct version number in api index

Previously, the index would show the version of Angular that runs on
the page, not the version for which the docs are. This meant that in
that snapshot docs the stable version was displayed.

The `$scope.docsVersion` value was used in the plnkr opening code, but
has not been used since https://github.com/angular/angular.js/commit/bdec35cebc89e0d80a04eeffbd71ad999fc7e61a.

Closes #15265