graphcast intro description needs ERA5 link fixing

[kiden]

In the opening description on https://github.com/google-deepmind/graphcast there's a sentence "Full model training requires downloading the ERA5 dataset, available from ECMWF."

Please can you update the first link in that sentence, i.e. the ERA5 URL, to https://www.ecmwf.int/en/forecasts/dataset/ecmwf-reanalysis-v5

Thanks