Fix code comment for Unpublished extension flag in extensionGalleryService.ts

[xavierdecoster]

The comment associated with this code is incorrect:

	/**
	 * This flag switches the asset uri to use GetAssetByName instead of CDN
	 * When this is used, values of base asset uri and base asset uri fallback are switched
	 * When this is used, source of asset files are pointed to Gallery service always even if CDN is available
	 */
	Unpublished = 0x1000,

I suggest changing it to something like this:

	/**
	 * The Unpublished extension flag indicates that the extension can't be installed/downloaded.
	 * Users who have installed such an extension can continue to use the extension.
	 */
	Unpublished = 0x1000,