fix: Fix a link in getting-started.mdx

[san-ghun]

Description (required)

Update getting-started.mdx file in src/content/docs/en/ to correct the link to be consistent with the content.

• The original content is linked to the Astro theme page, but suggest it would be better to be linked to the actual tutorial page in Astro docs.

[Build a Blog Tutorial]
(https://astro.build/themes/) --> (https://docs.astro.build/en/tutorial/0-introduction/)

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Name	Status	Preview	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	Dec 5, 2023 0:06am

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[ElianCodes]

Thanks for catching this! Just quickly updating the link here