Update Grafana k6 installation link in README.md

[MigrainePanda]

Pull Request

Description

Broken link to Grafana k6 installation page was updated to link to latest installation guide.

Fixes #344

Checklist:

Before you submit your Pull Request, please make sure you have completed the following tasks:

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.
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