[css-typed-om] CSSSkewY serialization typo

[arturkulig]

https://drafts.css-houdini.org/css-typed-om/#transformvalue-serialization

is:

• To serialize a CSSSkewY this:
  • Let s initially be "skewX(".

should be

• To serialize a CSSSkewY this:
  • Let s initially be "skewY(".