Update download-counts.md to correct the endpoint examples

[amalej]

Remove square brackets to correct the API example for downloads counts

The current example has square brackets, just removing those square brackets to correct the endpoint

https://api.npmjs.org/downloads/point/{period}[/{package}] > https://api.npmjs.org/downloads/point/{period}/{package}

References

• https://github.com/npm/registry/blob/main/docs/download-counts.md#point-values
• https://github.com/npm/registry/blob/main/docs/download-counts.md#ranges