[fx-masking] Example 17 uses to mask-mode: auto but that's not a valid value for mask-mode

[smfr]

https://drafts.fxtf.org/css-masking-1/#the-mask-mode lists the values for mask-mode as alpha, luminance and match-source.

However, Example 17, when talking about mask-type, has this example:

<p style="mask-image: url(#mask); mask-mode: auto;">
  This is the masked content.
</p>

I think that mask-mode should be match-source.

Also Example 17 has some formatting issues.