Create a function that takes a string txt and a number n and returns the repeated string n number of times.
If given argument txt is not a string, return Not A String !!
repeat_string("Mubashir", 2) ➞ "MubashirMubashir"
repeat_string("Matt", 3) ➞ "MattMattMatt"
repeat_string(1990, 7) ➞ "Not A String !!"