[filter-effects] Fix the 'tableValues' description of transfer functions

[RazrFalcon]

• https://www.w3.org/TR/filter-effects-1/#element-attrdef-fecomponenttransfer-tablevalues

For tableValues, the doc states that:

When type="table", the list of s v0,v1,...vn

But tableValues are used by the discrete type too.

For discrete, the function is defined by the step function given in the attribute tableValues