Fix day_4.py

Author: mariotristan

04_Day_Strings/day_4.py

@@ -168,7 +168,7 @@
 challenge = 'Thirty'
 print(challenge.isdigit()) # False
 challenge = '30'
-print(challenge.digit())   # True
+print(challenge.isdigit())   # True
 
 # isdecimal():Checks decimal characters