[0.5.0] docs: fix resolve.extensions format in blog post

[tobua]

In the new announcement blog post extensions (that are now explicitly required) are in the wrong format. I've fixed this with this pull request.

In the regular documentation extensions are already prefixed with a . so everything is fine there.

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🔨 Latest commit	177c60d685a9b195ad51e8f2dd5af8b752fe4239

[hardfist]

thanks