fix "Contexts in which this element can be used:" of <figcaption> element

[mtrootyy]

https://w3c.github.io/html/grouping-content.html#the-figcaption-element

Contexts in which this element can be used:

• As a descendant of a figure element.

↓
The above should be fixed following;

Contexts in which this element can be used:

• As a child of a figure element.

Ref: https://w3c.github.io/html/grouping-content.html#the-figure-element

Content model:

• Flow content optionally including a figcaption child element.