docs(modify_letter): remove reference to what to do on edge cases

subjects/modify_letter/README.md

@@ -8,10 +8,6 @@ Create a **function** `remove_letter_insensitive` that returns a string without
 
 Create a **function** `swap_letter_case` that returns a string swapping the case for the chosen letter.
 
-If there is an empty character in the arguments, the function should return the string as being passed.
-
-If there is an empty string, it should return an empty string.
-
 ### Expected Functions
 
 ```rust