Example markup for Secondary buttons shouldn't include the button class

[gordonmcmullan]

govuk-elements doesn't use .buton--secondary as a mixin but as a replacement for .button

Problem/Feature

Developers following our markup examples for v3.x markup are getting Green buttons instead of the expected grey buttons with the .button--secondary class. This is a change from v2.x when `class=button button--secondary' would have rendered the button as grey.

Expected behaviour

Secondary buttons should be Grey

Proposed solution

Change the markup examples and the example button to use only the button--secondary class