Blocknote Theming / Overriding CSS Docs minor update

[nicholasdavidbrown]

Relating to docs page: https://www.blocknotejs.org/docs/styling-theming/overriding-css

The data-level attribute doesn't appear to be on the header blocks anymore.

Current documentation: For example, .bn-block-content[data-content-type="heading"][data-level="2"] will select all heading blocks with heading level 2.

Updated to work at v0.22.0 Is now .bn-block-content[data-content-type="heading"] h2 etc. for each sub-element