Fix a typo in "Combination Sum"

On the problem's website :

https://oj.leetcode.com/problems/combination-sum/

the description of the problem had changed.

As the Note says, elements are in non-descending order, the greater-than sign
may lead to misunderstanding, and Leetcode had changed it to less-and-equal-than sign.

so I updated the content in the source file.