added DoubleCharacterSwap.cpp

[chiru-cd]

Description

Added DoubleCharacterSwap.cpp in medium category.

Program Name - DoubleCharacterSwap.cpp Issue No.- #129

Type issue number after hash(#)

Checklist(self check)

• [x] Yes, this task assigned to me.
• [x] My code follows the style guidelines(Clean Code) of this project
• [x] I have performed a self-review of my own code
• [x] My Filename is in camelCase format
• [x] I have added Problem statement, Problem link in file.

[chiru-cd]

@aayushi-droid This PR hasn't been merged yet. Please let me know if there is a problem with this request.