docs: refine docstring and simplify reverse_letters implementation

Updated the docstring for clarity and fixed formatting.

Author: Tithi234

strings/reverse_letters.py

@@ -1,7 +1,7 @@
 def reverse_letters(sentence: str, length: int = 0) -> str:
-    """
-    Reverse all words that are longer than the given length of characters in a sentence.
-    If unspecified, length is taken as 0
+    """Reverse all words longer than a given character length in a sentence.
+
+    If ``length`` is not specified, it defaults to 0.
 
     >>> reverse_letters("Hey wollef sroirraw", 3)
     'Hey fellow warriors'
@@ -13,7 +13,8 @@ def reverse_letters(sentence: str, length: int = 0) -> str:
     'racecar'
     """
     return " ".join(
-        "".join(word[::-1]) if len(word) > length else word for word in sentence.split()
+        word[::-1] if len(word) > length else word
+        for word in sentence.split()
     )