[docs] theme font sizes outdated

[oliviertassinari]

https://commerce.nearform.com/open-source/spectacle/docs/themes#theme-object

doesn't match with:

spectacle/packages/spectacle/src/theme/default-theme.ts

Lines 21 to 27 in 494c8b7

	fontSizes: {
	h1: '72px',
	h2: '64px',
	h3: '56px',
	text: '44px',
	monospace: '20px'
	},