fix: use correct yaml syntax in blog posts

[pngwn]

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[benmccann]

FYI @d3sandoval we'll need to be careful to put the title in quotes going forward to make it valid frontmatter (we're working on migrating the site to https://github.com/sveltejs/sites - the blog posts will still live here, but converting the markdown will be done differently after migration)

[DreaminDani]

Sounds great! Will add it to my template :)