add missing tf. in topk example

[moghammed]

Description

DOC
const a = tensor2d([[1, 5], [4, 3]]); -> const a = tf.tensor2d([[1, 5], [4, 3]]);

So the tf.topk example code works in the documentation

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[nsthorat]

Reviewed 1 of 1 files at r1.
Reviewable status: 0 of 1 approvals obtained