Exif function example, fix "metadata.downloadURLs" deprecated

[mxlius]

Fix error message: "Cannot read property '0' of undefined"
Root cause: Storage API FullMetadata.downloadURLs is deprecated, ref:https://firebase.google.com/docs/reference/js/firebase.storage.FullMetadata.html#downloadurls

Replace with Reference.getDownloadURL, https://firebase.google.com/docs/reference/js/firebase.storage.Reference.html#getdownloadurl

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[mxlius]

@googlebot I signed it!

[ramblescout]

Adding a note that I couldn't get this to work (node 12). I kept getting an error that downloads[0] wasn't an object. I switched back to the "deprecated" version and now it works. Might have trouble later, if so I'll come back and update.

Upload failed: - TypeError: undefined is not an object (evaluating 'snapshot.metadata.downloadURLs[0]')