Update day30.sol

Update day30.sol

day30: Concatenate two strings

day29: Create an array which stores addresses of the team players.

day28: Transfer amount

day27: Mapping

day26: Return the address of the owner of a contract

day25: Define an enum, write setter and getter functions for it

day24: Define an array within a struct, write getter and setter functions for it

day23: Define an array of struct, write getter and setter functions for it

day22: Define structs, write setter and getter functions

day21: Find the hcf of two numbers

day20: Return the second largest element in an array

day19: Return the sum of a series

day18: Return the sum of an array of numbers