Remove sr-only from components/dropdowns example

Assuming sr-only is used in the actual live code so that the menu can be
shown open already ... but having sr-only in the highlighted example
code itself is confusing/misleading.

Also, change the text for the button from "Dropdown" to "Dropdown
trigger" for clarity (and it then matches
http://getbootstrap.com/javascript/#dropdowns)
Remove sr-only from components/dropdown entirely

possibly less confusing option, as per discussion, is to always show the
dropdown trigger button (both live and in that code example)
Merge pull request #13461 from patrickhlauke/docs-dropdown-sr-only

Remove sr-only from components/dropdowns example