Update button-guidance.md to close <path> tags

[jonginn]

The <path> tags in the SVG logo in button-guidance.md are missing closing tags which makes them invalid HTML when users copy the code.

Found here: https://nhsconnect.github.io/nhslogin/button-guidance/#other

<path class="nhsuk-logo__background" fill="#fff" d="M0 0h40v16H0z" </path>
<path class="nhsuk-logo__text" fill="#005eb8" d="M3.9 1.5h4.4l2.6 9h.1l1.8-9h3.3l-2.8 13H9l-2.7-9h-.1l-1.8 9H1.1M17.3 1.5h3.6l-1 4.9h4L25 1.5h3.5l-2.7 13h-3.5l1.1-5.6h-4.1l-1.2 5.6h-3.4M37.7 4.4c-.7-.3-1.6-.6-2.9-.6-1.4 0-2.5.2-2.5 1.3 0 1.8 5.1 1.2 5.1 5.1 0 3.6-3.3 4.5-6.4 4.5-1.3 0-2.9-.3-4-.7l.8-2.7c.7.4 2.1.7 3.2.7s2.8-.2 2.8-1.5c0-2.1-5.1-1.3-5.1-5 0-3.4 2.9-4.4 5.8-4.4 1.6 0 3.1.2 4 .6" </path>

This PR makes the paths self closing:

<path class="nhsuk-logo__background" fill="#fff" d="M0 0h40v16H0z" />
<path class="nhsuk-logo__text" fill="#005eb8" d="M3.9 1.5h4.4l2.6 9h.1l1.8-9h3.3l-2.8 13H9l-2.7-9h-.1l-1.8 9H1.1M17.3 1.5h3.6l-1 4.9h4L25 1.5h3.5l-2.7 13h-3.5l1.1-5.6h-4.1l-1.2 5.6h-3.4M37.7 4.4c-.7-.3-1.6-.6-2.9-.6-1.4 0-2.5.2-2.5 1.3 0 1.8 5.1 1.2 5.1 5.1 0 3.6-3.3 4.5-6.4 4.5-1.3 0-2.9-.3-4-.7l.8-2.7c.7.4 2.1.7 3.2.7s2.8-.2 2.8-1.5c0-2.1-5.1-1.3-5.1-5 0-3.4 2.9-4.4 5.8-4.4 1.6 0 3.1.2 4 .6" />