Minor gRPC docs updates

[olegbespalov]

What?

It fixes a minor mistake on the gRPC protocol page, also linking the Protobuf JSON Mapping where we have examples and describes pitfalls.

Checklist

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Select one of these and delete the others:

If updating the documentation for the most recent release of k6:

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If updating the documentation for the next release of k6:

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Related PR(s)/Issue(s)

[heitortsergent]

Just some small edits, other than that LGTM! 🚢