make docs on how to use typography clearer

[sant0s]

** Is this for an existing documentation file? **

svelte-material-ui/packages/site/src/routes/demo/typography/_Typography.scss

Line 20 in 6ed73be

@use '@material/typography/mixins' as typography;

** What needs improvement? **

svelte-material-ui/packages/site/src/routes/demo/typography/_Typography.scss

Line 20 in 6ed73be

@use '@material/typography/mixins' as typography;

refers to @use '@material/typography/mixins' as typography; while https://sveltematerialui.com/TYPOGRAPHY.md refers to @use '@material/typography/index' as typography;

if one is preferred, please update the docs accordingly.