docs: contribution instructions teach how to run tests

[mikavilpas]

Fixes https://github.com/tree-sitter-grammars/tree-sitter-markdown/issues/136

^it looks like they ran into the same issue as me. It was difficult to figure out how to correctly run the tests, and I thought they are broken.

I found out the correct way to run them by meticulously reading these

• https://github.com/tree-sitter-grammars/tree-sitter-markdown/actions/runs/8272019685/job/22633037137
• https://github.com/tree-sitter/parser-test-action/blob/master/action.yml