docs: fix links on README

[gerardmarquinarubio]

I know this is an stupid PR but clicking any relative link from https://github.com/fastify/fastify README.md preview will take you to the desired href but not scroll you into view of the specified header in the link. Once you are already inside https://github.com/fastify/fastify/blob/main/README.md relative links will correctly scroll you into the header.

[gerardmarquinarubio]

I just wrote this PR because I have been learning Fastify and found it inconvenient to effectively double click every time I had to check something quickly in the readme. Sorry if I broke any contributing guidelines but it is such a simple fix that I couldn't help myself 😅

[climba03003]

The website of fastify is generated based on document file.
I think the change of relative link may break the website.

[gerardmarquinarubio]

The website of fastify is generated based on document file. I think the change of relative link may break the website.

So the README.md is used to generate the front-facing fastify.io website?

[climba03003]

So the README.md is used to generate the front-facing fastify.io website?

Yes

[luisorbaiceta]

We are not using the README @climba03003 See here

[climba03003]

We are not using the README @climba03003

So, it is the only piece that is not using for generating the website?
Then, I am OK to the changes.

[github-actions]

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