[egghead] Fixed egghead extractor - API base URL host updated

[bkvirendra]

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The Egghead extractor was raising the below exception:
[egghead:course] professor-frisby-introduces-composable-functional-javascript: Downloading course lessons JSON ERROR: Unable to download JSON metadata: HTTP Error 308: Permanent Redirect (caused by HTTPError()); please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; type youtube-dl -U to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.

The base URL in the extractor ('https://egghead.io/api/v1/series/%s/lessons') for fetching the JSON resource for the metadata was outdated.
I have gone ahead and updated the base URL to point to the updated base URL ('https://app.egghead.io/api/v1/series/%s/lessons').