Add exception for value 1

This subject was modified to fit the solutions (which is wrong), but it needs change back ones the solution is fixed. 1 has no prime factors.

subjects/fprime.en.md

@@ -10,6 +10,8 @@ Write a program that takes a positive `int` and displays its prime factors, foll
 
 -   The input, when there is one, will always be valid.
 
+-   In this exercise the primes factor of 1 is consider as 1
+
 ### Usage
 
 ```console
@@ -28,5 +30,7 @@ student@ubuntu:~/piscine-go/test$ ./test a
 
 student@ubuntu:~/piscine-go/test$ ./test 0
 
+student@ubuntu:~/piscine-go/test$ ./test 1
+1
 student@ubuntu:~/piscine-go/test$
 ```