Use literal value in json format example in readme

[gridbugs]

I imagine there is a tendency to copy this example when making one's first benchmark, and removing the obvious placeholders (the ellipses). The problem with including "value": "benchmark-value" in the format example is that it is valid json but isn't accepted by current-bench, so it may take a while for people to realize that this is the reason their benchmarks aren't producing graphs.

[ElectreAAS]

I didn't even know this could cause a problem. Nice catch and thanks for the PR!