docs: add missing mjpegScreenshotUrl in capabilities.md

[KazuCocoa]

appium-xcuitest-driver/lib/driver.js

Lines 463 to 467 in 1af61e5

	if (this.opts.mjpegScreenshotUrl) {
	this.log.info(`Starting MJPEG stream reading URL: '${this.opts.mjpegScreenshotUrl}'`);
	this.mjpegStream = new mjpeg.MJpegStream(this.opts.mjpegScreenshotUrl);
	await this.mjpegStream.start();
	}

Mostly copied from uia2 driver's one: https://github.com/appium/appium-uiautomator2-driver/blob/ce9e727f75509fd0b765ff394dca42e307d04515/README.md?plain=1#L158

[github-actions]

🎉 This PR is included in version 7.26.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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