typo fix Update cantor.md

[mdqst]

The typo in the text is in the sentence:

*"decimal expansion is not in the original list, the number $y$ is exactly the same as the number $x[2]$."

Corrected to:

*"decimal expansion is not unique in the original list, the number $y$ is exactly the same as the number $x[2]$."

This correction makes the sentence more logically consistent and accurate in the context of the mathematical argument.