doc: Fix diag() example to make is less confusing. (#7113)

Fixed an example which I found confusing because there's a 6 in the left column (at position [3][0][3][0]), until I noticed the typo, where the 6 is repeated instead of a 7.

tfjs-core/src/ops/diag.ts

@@ -38,7 +38,7 @@ import {op} from './operation';
  * tf.diag(x).print()
  * ```
  * ```js
- * const x = tf.tensor2d([1, 2, 3, 4, 5, 6, 6, 8], [4, 2])
+ * const x = tf.tensor2d([1, 2, 3, 4, 5, 6, 7, 8], [4, 2])
  *
  * tf.diag(x).print()
  * ```