fix go solution for "group-anagrams" to be optimal

[hiendevt]

fix go solution for "group-anagrams" to be optimal implement using letter count array as a key instead of a sorted string, according to the video solution.

• File(s) Modified: go/0049-group-anagrams.go
• Language(s) Used: go
• Submission URL: https://leetcode.com/problems/group-anagrams/submissions/949013175/

Important

Please make sure the file name is lowercase and a duplicate file does not already exist before merging.

[hiendevt]

Such a solution corresponds to the algorithm provided by the author and is more optimal.