Update syntax in EarthData examples

[kylebarron]

In https://developmentseed.org/obstore/latest/api/auth/earthdata/#obstore.auth.earthdata.NasaEarthdataCredentialProvider

Should ideally be

store.get

instead of obstore.get
.

And in the async example we should be using S3Store.from_url preferably instead of obstore.store.from_url.