Added doctest to detecting_english_programmatically.py (TheAlgorithms#11135)

Author: Suyashd999

strings/detecting_english_programmatically.py

@@ -25,6 +25,18 @@ def get_english_count(message: str) -> float:
 
 
 def remove_non_letters(message: str) -> str:
+    """
+    >>> remove_non_letters("Hi! how are you?")
+    'Hi how are you'
+    >>> remove_non_letters("P^y%t)h@o*n")
+    'Python'
+    >>> remove_non_letters("1+1=2")
+    ''
+    >>> remove_non_letters("www.google.com/")
+    'wwwgooglecom'
+    >>> remove_non_letters("")
+    ''
+    """
     return "".join(symbol for symbol in message if symbol in LETTERS_AND_SPACE)