Updated 404 broken link

[gaikwadrahul8]

Updated old link from :https://www.tensorflow.org/tutorials/customization/custom_training to new link :https://www.tensorflow.org/tutorials/customization/custom_training_walkthrough

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[gaikwadrahul8]

@ematejska created this PR to fix broken link so please kindly review it and do the needful.

[theadactyl]

@gaikwadrahul8 please sign the Google CLA when you get a chance. See message from CLA bot above

[gaikwadrahul8]

@ematejska, @theadactyl

Apologies for the delayed response and I have signed Google CLA so could you please help me to merge this PR ?

Thank you!