Replace attribute 'alt' with <title> for Mozilla logo

[xfq]

SVG uses the <title> element for alternative text.

Ref:

• https://css-tricks.com/accessible-svgs/
• https://www.sitepoint.com/tips-accessible-svg/
• https://github.com/mozilla/bedrock/blob/432a429f3d73eb60be65f36673a6be7e77d974c8/bedrock/base/templates/includes/global-nav.html#L16

[jdm]

Thanks!