Fix typo: "exhaution" to "exhaustion" in radix.go comments

[catatsuy]

This pull request includes minor changes to the radix.go file. The changes correct a typo in the comments across several functions. The word "exhaution" has been corrected to "exhaustion".

The changes were made in the following functions:

• Insert(s string, v interface{}) (interface{}, bool): Corrected the comment "Handle key exhaution" to "Handle key exhaustion".
• Delete(s string) (interface{}, bool): Corrected the comment "Check for key exhaution" to "Check for key exhaustion".
• Get(s string) (interface{}, bool): Corrected the comment "Check for key exhaution" to "Check for key exhaustion".
• LongestPrefix(s string) (string, interface{}, bool): Corrected the comment "Check for key exhaution" to "Check for key exhaustion".
• WalkPath(path string, fn WalkFn): Corrected the comment "Check for key exhaution" to "Check for key exhaustion".