fix(title): change default title to Rollup

[Haroenv]

This is how it's styled in the repo of Rollup itself (and already in the readme)

source: https://github.com/rollup/rollup

[btd]

Hi. I am fine with change, but you need to change also

rollup-plugin-visualizer/plugin/index.ts

Line 131 in 03a46f7

const filename = opts.filename ?? chooseDefaultFileName(opts);

[Haroenv]

Thanks, I didn't see the second result in my search for it. Updated!