fix bug of canot load EMNIST dataset #5356

[nonameR9077]

Description

The current MNIST URL - https://www.itl.nist.gov/iaui/vip/cs_links/EMNIST/gzip.zip is outdated which gives the NonMatchingChecksumError whenever tfds.image_classification.EMNIST is called.
Simply changed to working URL - https://biometrics.nist.gov/cs_links/EMNIST/gzip.zip which gives the same gzip.zip file.
This was first issued by davidshen84 here and found the working URL. Hurray for davidshen84!

[ccl-core]

Hi @nonameR9077 , thanks for your contribution!

Your PR is under review and will be merged soon.

For future reference notice that, when changing download links:

• Checksums should also be updated (https://www.tensorflow.org/datasets/add_dataset#checksums);
• The version should be increased, as well as the release notes.

[nonameR9077]

Checksums / versions / release notes, copy.
thank you for your fast review!

[ccl-core]

Thank you for the PR!