chore: update test command in contributing.md

[milo526]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[x] Other... Please describe: Update to contributing documentation

What is the current behavior?

The test commands as listed in the contributing.md are invalid. This might scare away developers who want to contribute but cannot test their contributions.

Issue Number: N/A

What is the new behavior?

The correct command is present in the contributing.md.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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