Update learn more link text in about:mullvad-browser

Would you mind updating the following source string from:

Curious to learn more about the browser? <a data-l10n-name="learn-more-link">Take a dive into the mole hole</a>.

to:

Curious to learn more about the browser? Read more at <a data-l10n-name="learn-more-link">mullvad.net</a>.