docs: fix dbStats example in contrib/database/sql

[mtoffl01]

What does this PR do?

The WithDBStats example on the contrib/database/sql go docs is inaccurate: https://pkg.go.dev/gopkg.in/DataDog/dd-trace-go.v1@v1.63.0/contrib/database/sql#example-package-DbStats
It's missing WithDBStats in the register function call. This PR updates the code snippet to an accurate example

Motivation

Clarity and usability for the feature

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-04-24 09:52:18

Comparing candidate commit 61c50fc in PR branch mtoff/dbstats-docs with baseline commit c4f2942 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 43 metrics, 1 unstable metrics.