docs: correct the message-handler guide's link label

[vaibhav8a]

docs/README.md:15 labels the guide \./message-handler-tunning-guide/`while linking tomessage-handler-customization-guide`.

Two things are off: tunning is misspelled, and more importantly the label names a directory that does not exist — the guide is message-handler-customization-guide, which is what the link target and the docs.axoniq.io URL on the same line both use.

Every sibling entry keeps label and target identical (\./identifier-generation-guide/`](identifier-generation-guide), `./meta-annotations-guide/`](meta-annotations-guide)`), so this one is the outlier. Corrected the label to match its target rather than only fixing the spelling.

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[smcvb]

@vaibhav8a, first and foremost, thanks for contributing!
I do need to ask though: can you sign the CLA? Without that, I am afraid I cannot merge this changes in.
The CLA is in place to protect both you and us against minor legal concerns.