docs(dql): Updated schema alter from date, to datetime

[alexatcanva]

Description

While following the https://dgraph.io/docs/dql/dql-get-started#step-4-alter-schema documentation I noticed that the described type for release_date was of type date not datetime.

It appears that in later versions of the Ratel UI (or perhaps it's always been?) date is now datetime.

This PR modifies the documentation from date to datetime.

Notes

This is my first contribution to the dgraph docs repo, please let me know if there's anything else I can do to make this a better PR!

Many thanks!

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