Doc: Fix is_prime (GH-9909)

Author: JulienPalard

Doc/library/concurrent.futures.rst

@@ -257,6 +257,10 @@ ProcessPoolExecutor Example
        1099726899285419]
 
    def is_prime(n):
+       if n < 2:
+           return False
+       if n == 2:
+           return True
        if n % 2 == 0:
            return False