docs(string_permutation): correct grammar

subjects/string_permutation/README.md

@@ -2,14 +2,9 @@
 
 ### Instructions
 
-Define the **function** `is_permutation` that returns true if:
+Define the **function** `is_permutation`, that returns `true` if the string `s1` is a permutation of `s2`.
 
-- the string `s1` is a permutation of `s2`, otherwise it returns false.
-- `s1` is a permutation of `s2` if all the elements in `s1` appear the same number of times in `s2` and all the characters in `s1` appear in `s2` even if they are in different order.
-
-### Notions
-
-- [hash maps](https://doc.rust-lang.org/book/ch08-03-hash-maps.html)
+`s1` is a permutation of `s2` if all the elements in `s1` appear the same number of times in `s2`, and all the characters in `s1` appear in `s2` even if they are in different order.
 
 ### Expected Function
 
@@ -44,3 +39,7 @@ $ cargo run
 Is `thought` a permutation of `thougth`? = true
 $
 ```
+
+### Notions
+
+- [hash maps](https://doc.rust-lang.org/book/ch08-03-hash-maps.html)