Refine docstring and simplify reverse_letters implementation (#14205)

* docs: refine docstring and simplify reverse_letters implementation

Updated the docstring for clarity and improved the logic for reversing words.

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* Update reverse_letters.py

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>

Author: 3 people

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
+    If ``length`` is not specified, it defaults to 0.
 
     >>> reverse_letters("Hey wollef sroirraw", 3)
     'Hey fellow warriors'
@@ -13,7 +13,7 @@ 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()
     )